Optical computational system

ABSTRACT

In optical filter systems and optical transmission systems, an optical filter compresses data into and/or derives data from a light signal. The filter way weight an incident light signal by wavelength over a predetermined wavelength range according to a predetermined function so that the filter performs the dot product of the light signal and the function.

This is a continuation of application Ser. No. 09/286,879 filed Apr. 6,1999, now abandoned the entire disclosure of which is incorporated byreference herein.

BACKGROUND OF THE INVENTION

The present invention relates to spectroscopy analysis systems. Moreparticularly, the invention relates to improvements in the compressionof data carried by light so that information about the light may beobtained.

Light conveys information through data. When light interacts withmatter, for example, it carries away information about the physical andchemical properties of the matter. A property of the light, for exampleits intensity, may be measured and interpreted to provide informationabout the matter with which it interacted. That is, the data carried bythe light through its intensity may be measured to derive informationabout the matter. Similarly, in optical communications systems, lightdata is manipulated to convey information over an optical transmissionmedium, for example fiber optic cable. The data is measured when thelight signal is received to derive information.

In general, a simple measurement of light intensity is difficult toconvert to information because it likely contains interfering data. Thatis, several factors may contribute to the intensity of light, even in arelatively restricted wavelength range. It is often impossible toadequately measure the data relating to one of these factors since thecontribution of the other factors is unknown.

It is possible, however, to derive information from light. An estimatemay be obtained, for example, by separating light from several samplesinto wavelength bands and performing a multiple linear regression of theintensity of these bands against the results of conventionalmeasurements of the desired information for each sample. For example, apolymer sample may be illuminated so that light from the polymer carriesinformation such as the sample's ethylene content. Light from each ofseveral samples may be directed to a series of bandpass filters whichseparate predetermined wavelength bands from the light. Light detectorsfollowing the bandpass filters measure the intensity of each light band.If the ethylene content of each polymer sample is measured usingconventional means, a multiple linear regression of ten measuredbandpass intensities against the measured ethylene content for eachsample may produce an equation such as:y=a ₀ +a ₁ w ₁ +a ₂ w ₂ + . . . +a ₁₀ w ₁₀  (Equation 1)where y is ethylene content, a_(n) are constants determined by theregression analysis, and w_(n) is light intensity for each wavelengthband.

Equation 1 may be used to estimate ethylene content of subsequentsamples of the same polymer type. Depending on the circumstances,however, the estimate may be unacceptably inaccurate since factors otherthan ethylene may affect the intensity of the wavelength bands. Theseother factors may not change from one sample to the next in a mannerconsistent with ethylene.

A more accurate estimate may be obtained by compressing the data carriedby the light into principal components. To obtain the principalcomponents, spectroscopic data is collected for a variety of samples ofthe same type of light, for example from illuminated samples of the sametype of polymer. For example, the light samples may be spread into theirwavelength spectra by a spectrograph so that the magnitude of each lightsample at each wavelength may be measured. This data is then pooled andsubjected to a linear-algebraic process known as singular valuedecomposition (SVD). SVD is at the heart of principal componentanalysis, which should be well understood in this art. Briefly,principal component analysis is a dimension reduction technique whichtakes m spectra with n independent variables and constructs a new set ofeigenvectors that are linear combinations of the original variables. Theeigenvectors may be considered a new set of plotting axes. The primaryaxis, termed the first principal component, is the vector whichdescribes most of the data variability. Subsequent principal componentsdescribe successively less sample variability, until only noise isdescribed by the higher order principal components.

Typically, the principal components are determined as normalizedvectors. Thus, each component of a light sample may be expressed asx_(n)z_(n), where x_(n) is a scalar multiplier and z_(n) is thenormalized component vector for the n^(th) component. That is, z_(n) isa vector in a multi-dimensional space where each wavelength is adimension. As should be well understood, normalization determines valuesfor a component at each wavelength so that the component maintains itshape and so that the length of the principal component vector is equalto one. Thus, each normalized component vector has a shape and amagnitude so that the components may be used as the basic buildingblocks of all light samples having those principal components.Accordingly, each light sample may be described in the following formatby the combination of the normalized principal components multiplied bythe appropriate scalar multipliers:x₁z₁+x₂z₂+ . . . +x_(n)z_(n).The scalar multipliers x_(n) may be considered the “magnitudes” of theprincipal components in a given light sample when the principalcomponents are understood to have a standardized magnitude as providedby normalization.

Because the principal components are orthogonal, they may be used in arelatively straightforward mathematical procedure to decompose a lightsample into the component magnitudes which accurately describe the datain the original sample. Since the original light sample may also beconsidered a vector in the multi-dimensional wavelength space, the dotproduct of the original signal vector with a principal component vectoris the magnitude of the original signal in the direction of thenormalized component vector. That is, it is the magnitude of thenormalized principal component present in the original signal. This isanalogous to breaking a vector in a three dimensional Cartesian spaceinto its X, Y and Z components. The dot product of the three-dimensionalvector with each axis vector, assuming each axis vector has a magnitudeof 1, gives the magnitude of the three dimensional vector in each of thethree directions. The dot product of the original signal and some othervector that is not perpendicular to the other three dimensions providesredundant data, since this magnitude is already contributed by two ormore of the orthogonal axes.

Because the principal components are orthogonal, or perpendicular, toeach other, the dot, or direct, product of any principal component withany other principal component is zero. Physically, this means that thecomponents do not interfere with each other. If data is altered tochange the magnitude of one component in the original light signal, theother components remain unchanged. In the analogous Cartesian example,reduction of the X component of the three dimensional vector does notaffect the magnitudes of the Y and Z components.

Principal component analysis provides the fewest orthogonal componentsthat can accurately describe the data carried by the light samples.Thus, in a mathematical sense, the principal components are componentsof the original light that do not interfere with each other and thatrepresent the most compact description of the entire data carried by thelight. Physically, each principal component is a light signal that formsa part of the original light signal. Each has a shape over somewavelength range within the original wavelength range. Summing theprincipal components produces the original signal, provided eachcomponent has the proper magnitude.

The principal components comprise a compression of the data carried bythe total light signal. In a physical sense, the shape and wavelengthrange of the principal components describe what data is in the totallight signal while the magnitude of each component describes how much ofthat data is there. If several light samples contain the same types ofdata, but in differing amounts, then a single set of principalcomponents may be used to exactly describe (except for noise) each lightsample by applying appropriate magnitudes to the components.

The principal components may be used to accurately estimate informationcarried by the light. For example, suppose samples of a certain brand ofgasoline, when illuminated, produce light having the same principalcomponents. Spreading each light sample with a spectrograph may producewavelength spectra having shapes that vary from one gasoline sample toanother. The differences may be due to any of several factors, forexample differences in octane rating or lead content.

The differences in the sample spectra may be described as differences inthe magnitudes of the principal components. For example, the gasolinesamples might have four principal components. The magnitudes x_(n) ofthese components in one sample might be J, K, L, and M, whereas in thenext sample the magnitudes may be 0.94J, 1.07K, 1.13L and 0.86M. Asnoted above, once the principal components are determined, thesemagnitudes exactly describe their respective light samples.

Refineries desiring to periodically measure octane rating in theirproduct may derive the octane information from the component magnitudes.Octane rating may be dependent upon data in more than one of thecomponents. Octane rating may also be determined through conventionalchemical analysis. Thus, if the component magnitudes and octane ratingfor each of several gasoline samples are measured, a multiple linearregression analysis may be performed for the component magnitudesagainst octane rating to provide an equation such as:y=a ₀ +a ₁ x ₁ +a ₂ x ₂ +a ₃ x ₃ +a ₄ x ₄  (Equation 2),where y is octane rating, a_(n) are constants determined by theregression analysis, and x₁, x₂, x₃ and x4 are the first, second, thirdand fourth principal component magnitudes, respectively.

Using Equation 2, which may be referred to as a regression vector,refineries may accurately estimate octane rating of subsequent gasolinesamples. Conventional systems perform regression vector calculations bycomputer, based on spectrograph measurements of the light sample bywavelength. The spectrograph system spreads the light sample into itsspectrum and measures the intensity of the light at each wavelength overthe spectrum wavelength range. If the regression vector in the Equation2 form is used, the computer reads the intensity data and decomposes thelight sample into the principal component magnitudes x_(n) bydetermining the dot product of the total signal with each component. Thecomponent magnitudes are then applied to the regression equation todetermine octane rating.

To simplify the procedure, however, the regression vector is typicallyconverted to a form that is a function of wavelength so that only onedot product is performed. Each normalized principal component vectorz_(n) has a value over all or part of the total wavelength range. Ifeach wavelength value of each component vector is multiplied by theregression constant a_(n) corresponding to the component vector, and ifthe resulting weighted principal components are summed by wavelength,the regression vector takes the following form:y=a ₀ +b ₁ u ₁ +b ₂ u ₂ + . . . +b _(n) u _(n)  (Equation 3),where y is octane rating, a₀ is the first regression constant fromEquation 2, b_(n) is the sum of the multiple of each regression constanta_(n) from Equation 2 and the value of its respective normalizedregression vector at wavelength n, and u_(n) is the intensity of thelight sample at wavelength n. Thus, the new constants define a vector inwavelength space that directly describes octane rating. The regressionvector in a form as in Equation 3 represents the dot product of a lightsample with this vector.

Normalization of the principal components provides the components withan arbitrary value for use during the regression analysis. Accordingly,it is very unlikely that the dot product result produced by theregression vector will be equal to the actual octane rating. The numberwill, however, be proportional to the octane rating. The proportionalityfactor may be determined by measuring octane rating of one or moresamples by conventional means and comparing the result to the numberproduced by the regression vector. Thereafter, the computer can simplyscale the dot product of the regression vector and spectrum to produce anumber approximately equal to the octane rating.

An example of a conventional spectroscopy analysis system is provided inFIG. 2. A laser 20 directs light to a sample 22 by a bandpass filter 24,beam splitter 26, lens 28 and fiber optic cable 30. Light is reflectedback through cable 30 through beam splitter 26 to a lens 32 to aspectrograph 34. Spectrograph 34 separates light from the illuminatedsample by wavelength so that the intensity of the light at eachwavelength can be measured by a detection device including a chargecouple detector 36. Charge couple detector 36 is controlled bycontroller 38 and cooled by cooler 40. The detection device measures thelight intensity of light from spectrograph 34 at each wavelength andoutputs this data digitally to a computer 42, which stores the lightintensity over the wavelength range. Computer 42 also stores apreviously derived regression vector for the desired sample property,for example octane, and sums the multiple of the light intensity and theregression vector intensity at each wavelength over the sampledwavelength range, thereby obtaining the dot product of the light fromthe substance and the regression vector. Since this number isproportional to octane rating, the octane rating of the sample isidentified.

Since the spectrograph separates the sample light into its wavelengths,a detector is needed that can detect and distinguish the relativelysmall amounts of light at each wavelength. Charge couple devices providehigh sensitivity throughout the visible spectral region and into thenear infrared with extremely low noise. These devices also provide highquantum efficiency, long lifetime, imaging capability and solid-statecharacteristics. Unfortunately, however, charge couple devices and theirrequired operational instrumentation are very expensive. A typicalinstrument including such sensitive detectors, and the spectrographsneeded for their operation, generally cost around $250,000. Furthermore,the devices are sensitive to environmental conditions. In a refinery,for example, they must be protected from explosion, vibration andtemperature fluctuations and are often placed in protective housingsapproximately the size of a refrigerator. The total cost for theseinstruments may range from $200,000 to $500,000. The power requirements,cooling requirements, cost, complexity and maintenance requirements ofthese systems have made them impractical in many applications.

SUMMARY OF THE INVENTION

The present invention recognizes and addresses the foregoingdisadvantages, and others, of prior art construction and methods.

Accordingly, it is an object of the present invention to provide animproved system for deriving information from light.

It is a further object of certain embodiments of the present inventionto employ an optical filter mechanism to optically compress data carriedby light into orthogonal components.

It is a still further object of certain embodiments of the presentinvention to optically compress data carried by light into components ofthe light so that data carried by said light may be measured bymeasuring a property of the components.

BRIEF DESCRIPTION OF THE DRAWINGS

A full and enabling disclosure of the present invention, including thebest mode thereof, directed to one of ordinary skill in the art, is setforth in the specification, which makes reference to the appendeddrawings, in which:

FIG. 1 is a graphical representation of an exemplary spectroscopicregression vector;

FIG. 2 is a schematic representation of a prior art spectroscopyanalysis system;

FIG. 3A is a schematic illustration of an optical analysis systemaccording to the present invention;

FIG. 3B is a schematic illustration of an optical analysis systemaccording to the present invention;

FIG. 3C is a graphical representation of the positive component of thespectroscopic regression vector as in FIG. 1;

FIG. 3D is a graphical representation of the negative component of thespectroscopic regression vector as in FIG. 1;

FIG. 4A is a schematic illustration of an optical analysis systemaccording to the present invention;

FIG. 4B is a schematic illustration of an exemplary filter device foruse in the system as in FIG. 4A;

FIG. 4C is a schematic illustration of an exemplary filter device foruse in the system as in FIG. 4A;

FIG. 5 is a schematic illustration of an optical analysis systemaccording to the present invention;

FIG. 6 is a schematic illustration of a summing circuit for summingweighted positive and negative light component portions;

FIG. 7A is a schematic illustration of the present invention utilizingan absorption spectroscopy method;

FIG. 7B is a schematic illustration of the present invention utilizingan emission or a scattering spectroscopy method;

FIG. 7C is a partial schematic illustration of the present inventionwherein light is directed to and from a sampled substance over opticalfiber;

FIG. 8A is a schematic illustration of an optical fiber splittermechanism for directing light from a sample to dual filters forseparately weighting positive and negative regression vector portions;

FIG. 8B is a schematic illustration of a beam splitter for directinglight from a sample to dual filters for separately weighting positiveand negative regression vector portions;

FIG. 8C is a schematic illustration of dual filters disposed toseparately weight positive and negative regression vector portions;

FIG. 9 is a schematic illustration of an optical analysis systemaccording to the present invention;

FIG. 10A is a graphical representation of a light pulse;

FIG. 10B is a graphical representation of the pulse of FIG. 10Afollowing optical fiber dispersion;

FIG. 10C is a schematic illustration of an optical analysis systemaccording to the present invention;

FIG. 11 is a schematic illustration of an optical analysis systemaccording to the present invention;

FIG. 12 is a schematic illustration of an optical analysis systemaccording to the present invention;

FIG. 13A is a schematic illustration of a photodiode array havingoptical filters disposed thereon;

FIG. 13B is a schematic illustration of a photodiode array havingoptical filters disposed thereon;

FIG. 14 is a schematic illustration of an embodiment of an opticalfilter according to the present invention

FIG. 15 is a schematic illustration of an optical analysis systemaccording to the present invention;

FIG. 16 is a schematic illustration of an optical analysis systemaccording to the present invention;

FIG. 17 is a schematic illustration of an optical analysis systemaccording to the present invention;

FIG. 18 is a schematic illustration of an optical analysis systemaccording to the present invention;

FIG. 19 is a schematic illustration of an optical analysis systemaccording to the present invention;

FIG. 20A is a graphical representation of an exemplary transmissionspectrum;

FIG. 20B is a graphical representation of the positive component of thetransmission spectrum shown in FIG. 20A;

FIG. 20C is a graphical representation of the negative component of thetransmission spectrum shown in FIG. 20A;

FIG. 20D is a graphical representation of a Fourier transform of thefunction shown in FIG. 20B;

FIG. 20E is a graphical representation of a Fourier transform of thefunction shown in FIG. 20C;

FIG. 21 is a schematic illustration of an optical analysis systemaccording to the present invention;

FIG. 22 is a partial perspective view of an embodiment of an opticalanalysis system according to the present invention;

FIG. 23 is a schematic illustration of an optical analysis systemaccording to the present invention;

FIG. 24A is a top diagrammatic view of an optical grating;

FIG. 24B is a side diagrammatic view of an optical grating;

FIG. 24C is a side diagrammatic view of an optical grating;

FIG. 24D is a schematic illustration of an embodiment of an opticalanalysis system according to the present invention and including a sidediagrammatic view of an embodiment of a grating-type optical filter; and

FIG. 25 is a schematic illustration of an optical analysis systemaccording to the present invention.

Repeat use of reference characters in the present specification anddrawings is intended to represent same or analogous features or elementsof the invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Reference will now be made in detail to presently preferred embodimentsof the invention, one or more examples of which are illustrated in theaccompanying drawings. Each example is provided by way of explanation ofthe invention, not limitation of the invention. In fact, it will beapparent to those skilled in the art that modifications and variationscan be made in the present invention without departing from the scope orspirit thereof. For instance, features illustrated or described as partof one embodiment may be used on another embodiment to yield a stillfurther embodiment. Thus, it is intended that the present inventioncovers such modifications and variations as come within the scope of theinvention.

In one presently preferred embodiment of an optical analysis system,shown generally at 44 in FIG. 3A, an energy source 46 illuminates asample substance 48. Light passing through or reflected from sample 48is collimated by collimator 50, which includes one or more lenses ormirrors to focus light from sample 48 into a parallel beam 49. Lightfrom collimator 50 is conveyed, for example through air, fiber opticcable, or other suitable medium, to optical filter 52. Optical filter 52is an interference device. That is, its performance depends upon thepath that light takes through it. Thus, collimator 50 directs a parallelbeam 49 to the filter. Light may be directed through a bandpass filterprior to optical filter 52 to eliminate light at wavelengths other thanthose encompassed by the regression vector.

Collimator 50 is not a spectrograph. Thus, the light in light beam 49 isunseparated, multiple wavelength light. Filter 52, however, is awavelength-specific light intensity filter. That is, the weighting itapplies to the light varies by wavelength. For example, suppose a lightbeam includes light at two wavelengths, 500 nm and 1000 nm, and that thelight intensity at 500 nm is G and the light intensity at 1000 nm is H.The total light intensity is G+H. A filter such as filter 52 may beconfigured to simultaneously filter the 500 nm light by 50% and the 1000nm light by 75%, even though the light at the two wavelengths arecombined parts of the same light beam. Accordingly, a light intensitydetector measuring the output of the filter would measure an intensityof 0.5G+0.75H.

In one preferred embodiment, an optical filter 52 includes multiplelayers of materials having different refractive indices. By properlyselecting the materials and designing the layer spacings, the filter canbe made to selectively pass predetermined fractions of light atdifferent wavelengths. Once the desired weighting at each wavelength isdetermined, the materials and spacings that compose optical filter 52may be determined using a variety of approximation methods. Thesemethods include, for example, determining the inverse Fourier transform(IFT) of the optical transmission spectrum and structuring the filter asthe physical representation of the IFT. The IFT suggests a continuousvariation of the refractive index within the filter structure. Atpresent, however, Applicants are unaware of a process for producing sucha continuously varying filter, and, therefore, further approximationsare used to convert the IFT into a usable structure based on knownmaterials with constant refractive indices. Such filters may be obtainedthrough the research group including George Dobrolski and Pierre Verlyunder the National Research Council of Canada. Information regarding thestructure of such filters is provided at Applied Optics, Vol. 35, pp.5484–5492 (1996) and Vol. 29, pp. 2876–2893 (1990).

In another preferred embodiment, a direct iterative process known as the“needle” method is used to construct the filter. This method begins withthe refractive indices of known materials and an estimate of the filterthickness. Through a computer algorithm, the effect of inserting“needles” of a second material into a first material is estimated. Theseneedles are then moved around within the second material, using theinterference pattern they create as a guide, until a best approximationof the desired interference pattern is produced. It should be understoodthat other suitable iterative methods may be used to produce the filter.

In one preferred embodiment of the present invention, the weightingsthat filter 52 applies at each wavelength are set to the regressionweightings b_(n) described with respect to Equation 3 in the Backgroundof the Invention. Thus, optical filter 52 optically performs the dotproduct of light beam 49 and a desired regression vector, and theintensity of light 54 output from optical filter 52 is directly relatedto the desired information. For example, if sample 48 is a gasolinesample, and if the regression vector embodied by filter 52 is an octaneregression vector for that particular gasoline type, the intensity oflight 54 is directly related to the octane of sample 48.

Accordingly, filter 52 simultaneously and optically perform twospectroscopic analysis steps. First, it compresses data carried by light49 into orthogonal components. Second, it weights the orthogonalcomponent magnitudes by regression vector weightings so that the outputof the filter is directly related to desired information.

Although, as discussed in more detail below, various types of orthogonalcomponents may be used, the orthogonal components in the octane examplemay be assumed to be principal components since the lightcharacteristics of the light source, the illuminated gasoline sample 48,are known. Since gasoline samples 48 measured by system 44 are similar,the regression constants a_(n) as in Equation 2 may be calculated andcombined to determine regression constants b_(n) as in Equation 3. Thus,optical filter 52 performs the dot product of the light 49 and theregression vector.

A detector 56 receives weighted light 54 from filter 52 and measures itsintensity. The measured intensity is the sum of the intensity at eachwavelength of light 54. As noted above, the intensity of light 54 isdirectly related to the actual measurement of the information associatedwith the regression vector. If the regression vector includes an offsetvalue a_(o) as in Equation 3, this may be introduced by a processor 58which receives the output from detector 56. The processor may also scalethe output, to account for normalization of the orthogonal componentsused to derive the regression vector as described above, so that thefinal output reflects an actual measurement of the desired information.The scaling may also be performed by one or more amplifiers followingthe detector, by an optical filter between filter 52 and detector 56, orby filter 52 if the scaling factor is incorporated into the wavelengthweightings. Processor 58 may be a stand-alone device or may beincorporated by the detector. It may comprise a microprocessor, forexample in a stand-alone computer, or digital and/or analog circuitry.It may also include a display meter to display the detector output in amodified or unmodified form and/or an output device so that the detectoroutput may be directed to external systems for processing. Where thereis no offset a_(o), or where all scaling is to be performed by anexternal system, the processor 58 may consist solely of a display meterand/or output device.

As noted above, detector 56 may be a conventional light detector, forexample constructed from germanium or silicon, for measuring theintensity of incident light. It should be understood, however, that anyother suitable light detector devices may be used, for example includingcameras or other film devices.

FIG. 3A illustrates a single optical filter 52. Since the optical filterof this embodiment is a transmission filter which passes a certainpercentage of incident light at each wavelength, it is unable to applynegative weightings. It is very unlikely, however, that the regressionvector will be entirely positive. That is, it is unlikely that eachregression constant b_(n) will be positive. To account for positive andnegative constants b_(n) of an exemplary regression vector 20 as in FIG.1, a system as illustrated in FIG. 3B includes a collimator 50 includinga pair of collimating lenses 51 a and 51 b directing light to filterdevices 52 a and 52 b, respectively. Filter 52 a is weighted with thepositive portion of the regression vector 20 (FIG. 1) as shown in FIG.3C, and filter 52 b is weighted with the negative portion as illustratedas in FIG. 3D. A pair of detectors 56 a and 56 b receive the outputlight 54 from filter 52 a and 52 b, respectively. Processor 58 sums thepositive output from detector 56 a with the negative output fromdetector 56 b to provide the dot product of the regression vector andthe light from sample 48. It should be understood that the output ofdetector 56 b is positive but that the output is summed to the detector56 a output as a negative number. That is, processor 58 subtracts theoutput of detector 56 b from the output of detector 56 a.

The regression vector constants b_(n) are most likely between −1 and 1and are likely to be relatively close to 0. Since, in the embodiment ofoptical filter 52 illustrated in FIGS. 3A and 3B, these numbersrepresent percentages of incident light passed to the detectors, thesignal-to-noise ratio may be improved by unitizing the regression vectorconstants. That is, the constant b_(n) having the largest absolute valueis scaled to 1 or −1, depending on whether the constant is positive ornegative. All the other constants b_(n) are scaled by the same factor.These scaled constants then become the weightings b_(n) by which filter52 weights incident light at each wavelength. The output from the filteris then reduced by this scaling factor in the manner described aboveregarding the scaling factor caused by the use of normalized orthogonalcomponents. That is, the regression vector's unitization modifies thescaling factor resulting from regression vector normalization.

Although unlikely, it is possible that one or more of the constantsb_(n) of Equation 3 may be greater than 1 or less than −1. Although itis possible to use optical filter mechanisms, such as are describedbelow, which are able to amplify light at different wavelengths, ascaling factor less than 1 may be used to reduce the constant b_(n) sothat the greatest magnitude constant is 1 or −1. The unitized constantsmay then be used by an optical filter such as filter 52 described above.

Those of ordinary skill in this art should understand that energy source46 may include various suitable energy sources, for example as are usedin known spectroscopy methods. For example, referring to FIG. 7A, energysource 46 may include a broad band light source 47 proximate sample 48.Exemplary broad band light sources include lamps, filaments, LEDs, orother devices providing multi-wavelength light substantially over thevisible and near visible light spectrum. One or more broad band sourcesmay be positioned proximate sample 48 so that light emanating from thelight source is directed by lens 53 to bandpass filter 55, which limitsthe light to a wavelength range equal to or within the regression vectorwavelength range, and then on to the sample. This light may then passthrough, as shown in FIG. 7A, or reflect from the sample to be analyzeddownstream by optical system 57 to derive desired information about thesample. Optical system 57 includes an optical filter mechanism such asillustrated in FIG. 3B. Examples of such absorbance spectroscopy methodsinclude infrared absorbance, near infrared absorbance(NIR), mid infraredabsorbance(MIR) and ultraviolet visible absorbance(UV-VIS).

Energy source 46 may also illuminate sample 48 by exciting the sample sothat it emits light. Such energy sources may include lasers, lamps orelectricity sources. For example, referring to FIG. 7B, a laser or lamp47 emits light that is filtered and directed to sample 48 by lenses 53 aand 53 b and bandpass filter 55. Sample 48 is excited so that it emitslight, which is directed by lens 53 c to system 57 for further analysis.Examples of such spectroscopy methods include fluorescence emission,phosphorescence emission, luminescence emission and electroluminescence.

FIG. 7B may also be used to illustrate scattering methods in whichenergy source 46, which typically includes a laser 47, exposes sample 48to monochromatic light. As light passes through the sample, it isscattered into various wavelength bands. Examples of such methodsinclude Raman scattering, Mie scattering, Rayleigh scattering andquasi-elastic light scattering. The configuration illustrated in FIG. 7Bmay be used in both emission methods and scattering methods, primarilybecause luminescence and scattering effects normally coexist, and thedifference between these method types is largely a matter of analyzingthe output. Certain circumstances, for example the wavelength of theenergy used to illuminate the sample, or the sample itself, may causeone effect to dominate ever the other.

It should be understood that any of the above-described, or othersuitable illumination methods may be employed with the presentinvention. Those of ordinary skill in this art should understand suchmethods and systems, and further detailed explanation thereof istherefore not provided.

In the examples illustrated in FIGS. 7A and 7B, light is conveyed to andfrom sample 48 over air. It should be understood, however, that otherlight media may be used, for example fiber optic elements 59 asillustrated in FIG. 7C.

A spectroscopic regression vector is affected by the instruments andmethods used to drive it. Thus, once a regression vector has beenestablished with certain equipment, an equipment change may require acalibration of the system. To effect a calibration without recalculatingthe regression vector, filters may be placed between the filters 52 a,52 b and detectors 56 a, 56 b shown in FIG. 3B. Alternatively,adjustable amplifiers may adjust the detector output, or processor 58may be reprogrammed or recalibrated. Calibration, and other systemadjustments, may be easily effected where the system is configured toexecute the regression vector equation as in Equation 2. A series ofoptical filter pairs, such as filters 52 a and 52 b in FIG. 3B, may beused to separate the orthogonal components from the light sample.Amplifiers may then apply the regression components a_(n) as in Equation2 to the output of the detectors following each optical filter pair. Ifthe amplifiers are adjustable, the regression vector constants may beadjusted directly to account for the calibration. This type ofarrangement may also be used where a product change requiresredetermination of the regression vector. For example, a change in arefinery's manufacturing process or ambient conditions may change theregression vector. The adjustable amplifiers permit the change withoutrequiring new optical filters. While the Equation 2 arrangement requiresmore filters than the Equation 3 arrangement, the expense may bejustified if the regression vector is likely to change. A more detaileddescription of an Equation 2 arrangement is provided below.

It should also be understood that the shape and magnitude of theregression vector will depend upon the method used to derive it. Forexample, in terms of frequency, Raman spectroscopy produces sharp (forexample, 10 cm⁻¹) peaks, while near infrared spectroscopy producesbroader (for example 100 cm⁻¹) peaks, and UV-VIS spectroscopy producesvery broad (for example, 500 cm⁻¹) peaks. Thus, the wavelengthdistribution is different for the different types of spectroscopy.Although the magnitudes may differ, all regression vectors may be scaledto unit weightings, for example through appropriate filterconfiguration. As noted above, subsequent electronics may providecorrection via an amplifier or other suitable device. Although it may bepossible to use different spectroscopy methods in deriving theregression vector and in performing the subsequent analysis if thefilter is properly configured to account for the difference, it ispreferred to use the same spectroscopy method for both.

Light from sample 48 may be directed to lenses 51 a and 51 b by any ofseveral suitable methods. Referring to FIG. 8A, for example, light maybe conducted from the sample to the lenses 51 a and 51 b over fiberoptic cable 80 constructed in a bundle branching into two sections.Light striking the combined face of the fiber bundle is separated intotwo divisions 80 a and 80 b which approach the lenses. Alternatively,referring to FIG. 8B, a beam splitter 82 may be used to separate thelight, as should be understood by those of ordinary skill in this art.The beam splitter assembly could be neutral, splitting every wavelengthnearly equally into two directions, or dichroic, sending somewavelengths in one direction and others in another. In anotherconfiguration, light is split by the disposition of the filters 52rather than by an upstream device. Thus, a single lens collimator 50 isused to direct light from the sample to the filters.

The first of the filters, for example filter 52 a in FIG. 8C, isdisposed to operate at a slight angle, for example 10°, with respect tothe path of light 49. Detector 56 a (not shown in FIG. 8C) is disposedbeyond filter 52 a to receive the light transmitted by the filter. Thelight of all wavelengths not passed by filter 52 a is reflected at anangle of 20° from the path of light 49. Filter 52 b is disposed tooperatively receive this light, and detector 56 b (not shown in FIG. 8C)is disposed beyond filter 52 b to receive the weighted light therefrom.Processor 58 (not shown in FIG. 8C) measures the output of the twodetectors as described above. Since filters 52 a and 52 b never haveoverlapping transmission, the reflected light from the first can feedthe second filter directly, obviating the need to separate the lightprior to filtering.

One exemplary embodiment of processor 58 is schematically illustrated inFIG. 6. Output signals from detector 56 a and 56 b are provided to aresistor and op-amp circuit 76. Since the outputs from the detectors arepositive, the summing circuit performs a subtraction function.Specifically, the output to display device 78 is given by the equationv _(o)=(R3/R1)((R1+Rf)/(R2+R3))56a−(Rf/R1)56b.By appropriately selecting the resistor values, the amplifier gains, ifdifferent, can be compensated and the correct substraction performed.

Various suitable filter mechanisms other than the filter devicesdescribed above with respect to FIG. 3B may be used for optical datacompression. Although the discussion below provides examples of suchmechanisms in the context of an Equation 3-type regression vectorarrangement, it should be understood that such mechanisms may be appliedin other embodiments described herein.

Accordingly, in the embodiment illustrated in FIG. 4A, an optical filtermechanism includes a pair of spectrographs 60 a and 60 b and a pair ofoptical filters 64 a and 64 b. Light from sample 48 is directed tospectrographs 60 a and 60 b, which output light spectra 62 a and 62 b tothe filters. The operation and construction of a spectrograph should bewell known to those of ordinary skill in this art and are, therefore,not described in detail herein. In general, however, each spectrographincludes a narrow input slit of a given height and produces an outputspectrum much broader than the input slit but having the same height.The light intensity varies laterally across the spectrum by lightwavelength.

As discussed above regarding the system illustrated in FIG. 3B, thesystem of FIG. 4A separately weights the positive and negativeregression vector constants. Thus, the system includes two filters 64 aand 64 b, weighted with the positive and negative constants,respectively. Although configured to the different weightings, thestructure and general operation of the two filters is the same.Accordingly, only filter 64 a is illustrated in FIG. 4B. Referring toFIG. 4B, filter device 64 a includes a plurality of areas 66 arranged sothat each area 66 receives light of a particular wavelength in spectrum62 a. Since areas 66 have a certain width, each receives light fromspectrum 62 a over a certain wavelength range. However, the range issmall and may be considered a single wavelength as used herein.

The total spectrum wavelength range depends, for example, on thespectroscopy method used. For example, Raman typically covers a morenarrow range than NIR. Also, some sections of the optical spectrum maycontain more information than other sections. Thus, some spectrumsections may be omitted from the regression vector to improveperformance.

Filter device 64 a weights the intensity of light at each wavelength inspectrum 62 a as determined by the spectroscopic regression vector. Theweightings may be effected in various suitable fashions. For example,each area 66 may include a plurality of light sensor devices 68, forexample including liquid crystal display devices (LCDs) or fiber opticelements. Each device 68 detects the presence or absence of incidentlight. Thus, the weighting at any given area 66 may be determined byselecting the number of devices 68 which will be measured. For example,a certain number of devices 68 may be deactivated, or a control systemmay selectively monitor the output of a predetermined number of thedevices. Weighting may also be accomplished by selecting the density ofthe sensor devices 68 over the various wavelengths. Again, theweightings at a given “wavelength” will be applied over a certain range.For example, since fiber optic elements are approximately 0.3 nm wide inwavelength space, the wavelength range of a particular area isapproximately 0.3 nm.

Filter 64 a and an associated control system may comprise a single unitwhich weights light from spectrograph 60 and detects the weighted light.Such a control system may include a computer device 70 for controllingthe operation of the filters and monitoring their output.

In another preferred embodiment, filter devices 64 a and 64 b may eachinclude an array of transmission filters configured to selectively passlight to detector devices 56 a and 56 b, respectively. Referring to FIG.4C, each area 66 of filter 64 a includes a plurality of transmissionfilters 72 so that the amount of light measured by detector 56 a at eachwavelength along the spectrum 62 a is determined by the number oftransmission filters which pass light at each area 66. The number oftransmission filters passing light at each area is determined by theweightings of the regression vector. The filters 72 may include, forexample, adjustable shutters which may be selectively opened or closed.Thus, the number of open or closed shutters in a given area 66determines the percentage of light passed from that area to the detector56 a.

Alternatively, filters 72 may be constructed so that each passes apredetermined percentage of incident light, thereby causing the area 66to pass a predetermined percentage. By including the proper suchfilters, which may include, for example, photographic film plates orholographic optical elements, the light passing percentages at each area66 may be set to the regression vector weighting at the relevantwavelength.

Filters 64 a in FIGS. 4B and 4C may be adjustable so that the weightingsat each wavelength may be changed to accommodate a new regressionvector. For example, a number of sensors 68 may be selectivelyactivated, deactivated or monitored as needed, for example by computerdevice 70. Similarly, transmission filters 72 may be activated,deactivated or otherwise configured, for example manually or by computer70, to pass or reject light as needed.

In another preferred embodiment illustrated in FIG. 5, light from sample48 is weighted by acoustooptical filters 73 a and 73 b. Again, twofilters are used to accommodate positive and negative regression vectorconstants. Each filter 73 a and 73 b passes light at a single wavelengthat a time, the wavelength being determined by acoustic wave controlsignals from computer 74. The filters weight the light from sample 48 byvarying the time over which light is passed at each wavelength. Bysetting the relative time periods at each wavelength to correspond tothe relative weightings at each wavelength in the regression vector, thefilters weight the light in a pattern that corresponds to the regressionvector. Light detector devices 56 a and 56 b measure the intensity oflight passed from filters 73 a and 73 b as they scan through theapplicable wavelength range. Processor 58 sums the positive componentfrom detector 56 a with the negative component of detector 56 b.Alternatively, filters 73 a and 73 b may be liquid crystal tunablefilters. The operation of crystal tunable filters, as should beunderstood by those of ordinary skill in this art, is similar to that ofacoustooptical filters.

While the acoustooptical filters are described above as passing light ata particular wavelength, it should be understood that light is passedover a relatively small wavelength range. For example, an acoustoopticalfilter may have a bandwidth near 10 nm. As discussed herein, however,these ranges may be considered single wavelengths for a givenapplication.

While the embodiments of the present invention discussed above weightthe intensity or, similarly, time of light intensity exposure of lightfrom the sample substance, it should be understood that other propertiesof light may be weighted. For example, light polarization or coherencemay be weighted as a function of wavelength.

The present invention may be used to optically compress light data inapplications in which the regression vector constants may change fromsample to sample and in applications not suitable for principalcomponent analysis. For example, a class of substances may produce thesame principal components but not be subject to a unique regressionvector. Gasolines again provide an example. All gasolines are composedof the same major compounds, but a high octane gasoline may be obtainedin various ways by mixing different compounds. Particularly, mixturesmay vary from manufacturer to manufacturer. Principal component analysisperformed on sample gasoline spectra from different manufacturers mayreveal that the gasolines have the same, or very similar, principalcomponents but that the relative importance of the components differsamong the manufacturers. Consequently, the regression vector of thegasoline of one manufacturer will be different from that of another.Another example is the prediction of blood glucose levels using lighttransmitted through blood samples. A regression vector may be determinedfor an individual to relate the blood principal components to bloodglucose level. From this point on, this persons blood glucose level maybe monitored optically. If the same instrument is used on anotherperson, however, the glucose measurement may fail, even though theprincipal components for the two individuals are the same. Differencesbetween the individuals, for example race, blood type, or weight, maycause the weightings of the principal components to differ sosignificantly that the regression vector for the first individual is notapplicable to the second.

An optical analysis system 44 illustrated in FIG. 9 may be effectivelyused in these situations to optically compress light data fromnon-similar light sources to derive desired information. An energysource 46 illuminates a sample, for example a blood sample, 48 by anysuitable method as described above. Light from illuminated sample 48 isconveyed by suitable means to a series of collimators 50 which directthe light in parallel beams 49 to a series of optical filter devices.The optical filters may be constructed, for example, as the opticalfilters 52 a and 52 b discussed above regarding FIG. 3B. The opticalfilters are grouped in pairs, each pair corresponding to a principalcomponent applicable to the sample, in this case human blood. Thus,optical filters 52 a ₁ and 52 b ₁ may correspond to the positive andnegative portions, respectively, of the first principal component.Optical filters 52 a ₂ and 52 b ₂ correspond to the second principalcomponent. Additional filter pairs follow up to filters 52 a _(n) and 52b _(n), where n is the number of principal components.

The arrangement as in FIG. 9 may be used to derive information from alight sample using the regression vector format as in Equation 2. Sincea regression vector for one sample is inapplicable to other samples,there are no regression constants a₁ through a_(n) that can be used foreach blood sample. Thus, in constructing the optical filters, theseconstants are assumed to be 1. The optical filter weighting percentages,therefore, correspond to the values of the normalized principalcomponents at each wavelength. For example, the normalized firstprincipal component may have a value of −0.04 at 500 nm. Thus, theweighting percentage of optical filter 52 b ₁ at 500 nm is 4%, and theweighting of optical filter 52 a _(n) at the same wavelength is 0. Thecomponents may also be unitized to improve the signal-to-noise ratio.Accordingly, each optical filter pair performs the dot product of thelight from the light source with its respective principal componentvector, and the amount of light output from the optical filter pair isproportional to the contribution of that principal component to theoriginal light from the sample. Again, the proportionality is due to thenormalization and unitization of the component vector. This may beaccounted for by processor 58, which sums the output of the lightdetectors 56 a and 56 b or by a downstream computing device 82.

The optical filter mechanism of FIG. 9 compresses the data carried bythe light from the light source into principal components in a manner sothat the principal component magnitudes may be separately detected.Accordingly, a display device such as an LED (not shown) may be attachedto the output of each processor 58. Assuming that the proportionalityfor each component is accounted for in some manner prior to theprocessor output, such devices display the magnitude of each componentin the original light.

This system may be used to accurately measure the blood glucose level ofany individual. For example, several blood samples may be drawn from anindividual and analyzed by the system to determine the magnitude of eachprincipal component in each sample. This data may be directed to acomputer 82, or other suitable device, which performs a multiple linearregression of the component magnitudes for each sample against the bloodglucose level for each sample measured by conventional means. Theregression produces the regression constants a₀ through a_(n) ofEquation 2. The constants may be applied to the output from subsequentblood samples by computer 82 so that the system may be used toaccurately measure blood glucose for this individual.

In another embodiment, computer 82 is not used in measuring subsequentblood samples. Instead, a series of adjustable amplifiers 84 apply again to the detector outputs equal to the respective regressionconstants a₁ through a_(n). A summing circuit, which may include anysuitable mechanism such as a microprocessor or summing circuitry, may beused to sum the output of each amplifier 84. The output of the summingdevice may then be offset by the constant a₀ and scaled by a scalingfactor as described above by an appropriate amplifier or computingdevice. The offset mechanism may be any suitable device which, forexample, adds or subtracts an appropriate DC offset to or from thedetector output. The offset mechanism and the scaling factor amplifiermay also be adjustable.

Accordingly, the system as in FIG. 9 may be used to analyze any samplehaving the principal components embodied by the optical filters. It maybe used to determine the regression vector constants for similar samplesand thereafter, by appropriately setting the adjustable gains andoffset, to apply the appropriate regression vector for those samples.Such a system may also be used where a regression vector is known tochange over time. Thus, rather than constructing a single optical filterpair to perform the dot product according to the Equation 3 regressionvector, a refinery may use an adjustable configuration as shown in FIG.9 so that the regression vector may be changed as needed.

It should also be understood that the system illustrated in FIG. 9 maybe constructed in various suitable forms. For example, various suitablecombinations of computing devices and/or amplifiers and/or opticalfilters may be used to effect the gains, offsets and summations appliedand performed by the system. For example, the detector outputs may allbe directed to a computer which may perform all of these functions.Furthermore, the system may be packaged in a relatively small kit withan energy source capable of illuminating a blood sample so that, oncethe appropriate gains and offset are set, an individual may performglucose testing at home. Principal components are merely one set oforthogonal components. They are very useful in that they represent themost compact form of data compression. That is, they represent thefewest number of orthogonal components that completely describe the datain the original signal. If light is to be analyzed from sources havingdifferent principal components, however, principal component analysis isnot an effective data compression tool. Thus, it should be understoodthat the filter mechanism of the present invention may compress lightdata into orthogonal components other than principal components, forexample Fourier components or wavelet components. Like principalcomponents, these other component types are orthogonal components of theoriginal waveform. They are not, however, as efficient as principalcomponents, and more components are necessary to adequately describe thelight data. For example, each Fourier component has the shape of a sinewave. Thus, the Fourier components are a series of sine waves ofdifferent magnitudes and frequencies which, when combined, produce theoriginal signal. Each sine shape may be normalized and/or unitized sothat each component may be used as weightings for an optical filter pairsuch as filters 52 a ₁ and 52 b ₁ in FIG. 9. Thus, the system in FIG. 9could be configured to compress the light data into Fourier components.

Embodiments of the present invention using non-principal component datacompression may be used in a variety of applications. For example, thepresent invention may be used with satellite systems that receive lightreflected from the earth and relay light data to the earth for analysisto derive desired information, for example the location of oil deposits.In this case, the light source is light reflected from earth rather thanlight from an illuminated sample substance. Light reflected fromdifferent parts of the earth may have very different principalcomponents, but may still be affected by oil deposits beneath thesurface. A series of optical filters may be constructed to compress thedata carried by this light into a predetermined set of orthogonalcomponents, for example Fourier or wavelength components. An opticalfilter pair, such as illustrated in FIG. 9, may be constructed for eachcomponent and housed in the satellite. The earth's surface may be imagedthrough each filter pair in turn to provide a compact data setcontaining all the pertinent spectroscopic data that can be obtained.Thus, the satellite carries an optical filter for each significantcomponent and, for example, a separate camera for each optical filter orone camera that sequentially views the earth's surface through eachfilter. This compact information may be transmitted to a processing uniton earth, where the spectra can be reconstituted from the compactrepresentation of Fourier or wavelet components. The number ofcomponents used may be chosen to provide a desired spectroscopicresolution. That is, the more components used, the more accurately theactual spectrum may be recreated. Thus, earth-based researchers may haveat their disposal all the significant optical information about theearth's surface.

The present invention may also be used in systems such as communicationsystems where information is stored in predetermined orthogonalcomponents. Light comprising the components is transmitted through anoptical medium to a set of filters configured to compress the light tothe components to derive the information. By using orthogonalcomponents, multiple signals may be transmitted simultaneously withoutinterference.

Such a system is particularly advantageous in long range opticalcommunications systems that transmit information in wavelength-divisionmultiplexing (WDM).

In current communications systems, the use of WDM is limited by overlapof neighboring wavelength channels. For example, if the closest spacingof WDM channels without overlap is 10 nm, and if the wavelength band isapproximately 100 nm, at most 10 channels may be simultaneouslytransmitted. Using the present invention, a far greater number ofchannels may be arranged as orthogonal components in wavelength spaceand combined to be simultaneously transmitted over an optical medium, atthe end of which a group of optical filters compresses the data into theorthogonal components for measurement. Channel overlap does not damagedata transmission integrity because the channels are orthogonal to oneanother.

In one preferred embodiment, each channel takes the form of somepredefined signal, for example a pulse having some shape. The pulses areorthogonal to each other. To create orthogonal pulses, each pulse isconfigured so that if each is plotted as a function of wavelength, thedot product of any pulse with any other is zero. Any number of thesepulses may be transmitted over the optical medium, for example fiberoptic cable, to a series of optical filters. Thus, one or more opticaltransmission line is the light source for the optical filters. Eachoptical filter pair in the series may be configured as described aboveso that it performs the dot product of the light from the fiber opticcable with a particular one of the orthogonal shapes. Thus, the opticalfilter series compresses the light data into predefined transmissionchannel components.

Each orthogonal shape may be normalized and/or unitized so that anoptical filter such as described above regarding FIGS. 3 and 9 may beused. If a pulse corresponding to any particular filter pair is notpresent in the light signal, the output from the detectors correspondingto this filter pair is zero. If a pulse is present, the detectorsmeasure a value dependent upon the magnitude of the pulse in the signal.Since this magnitude may be predetermined, the system may be configuredto look for a particular level to identify the presence of a pulse. Inthis way, a far greater number of pulses may be conveyed and recognizedover optical media than is possible in conventional systems.

The limit to the number of orthogonal signals may be determined by themodulation frequency of the channel. For example, a 10 GHz data rate onone channel blurs its spectrum by 10 GHz. If Fourier functions are used,the spacing between crests of the most complex filter will not be closerthan 10 GHz. This represents the maximum theoretical data transmissionrate—approximately 10 to 100 times greater than conventional WDMsystems. The Fourier functions may be constructed from etalons, whichhave sine wave spectra. An etalon is analogous to a pair of mirrors witha spacer between them. Different spacer thicknesses provide differentsine functions. An orthogonal system may use WDM channels bytransmitting orthogonal functions inside each WDM channel.

As indicated above, the present invention may be utilized in a varietyof environments. For example, the process of reading gel electrophoresisplates in conventional genetics testing is relatively time and laborintensive. Using the present invention, light passed through aphotograph of a gel electrophoresis plate may be directed to opticalfilters which compress the light data into orthogonal componentscorresponding to particular taggants.

Negative Dispersion Filter to Counteract Dispersion in Optical Fibers

Optical filters may affect the phase of light passing through them, andmay do so differently at different wavelengths. That is, due to theconfiguration of the materials comprising the filter, the filter maypass light at different wavelengths at different speeds. Accordingly,the filter creates a phase shift that varies with wavelength over thewavelength range of the incident light. While phase shift is of littleor no concern in certain embodiments of the present invention, opticalfilters may be used in other environments to counteract wavelengthdependent phase shift effects present in optical systems.

Fiber optic communications systems provide one such environment. Asignal in any medium travels more slowly than the speed of light. Asshould be understood in this art, a light signal's speed in an opticalfiber depends on the material's refractive index. An optical fiber'srefractive index, however, varies with the light signal's wavelength,causing the fiber to disperse the signal. That is, because the lightsignal travels at different speeds at different wavelengths, there is arelative phase shift over the signal's frequency range.

Referring to FIG. 10A, a light signal pulse 90 is defined by threecoordinates I, λ and t representing light intensity, wavelength andtime, respectively. As shown in the figure, the square-shaped lightsignal 90 is pulsed on for a time T on a channel that extends at leastfrom λ_(A) to λ_(B).

Because the signal travels down a fiber at different speeds over itswavelength range λ_(A)–λ_(B), the fiber disperses the signal, forexample as shown in FIG. 10B, by the time the signal reaches the fiber'send. As shown in FIG. 10B, light at wavelength λ_(B) traveled at thefastest speed, while light at wavelength λ_(A) traveled at the slowest.Light at intermediate wavelengths traveled at speeds between theseextremes, resulting in the dispersed signal shown of FIG. 10B. Theshapes of the signals in FIGS. 10A and 10B are provided by way ofexample only and do not represent actual signals.

The relative phase shift between light at any two wavelengths ispredictable. The speeds S_(A) and S_(B) of light at wavelengths λ_(A)and λ_(B), for example, may be determined from the fiber's refractiveindex at each wavelength, as should be understood by those skilled inthis art. Assume that speeds S_(A) and S_(B) are in units of nm/sec andthat the fiber's length is L, in nm. Light at wavelength λ_(B) travelsthe length of the fiber in L/S_(B) seconds. In this same time period,light at wavelength λ_(A) travels S_(A)(L/S_(B)) nm. The shift D betweenlight at λ_(A) and light at λ_(B) is, therefore, L−S_(A)(L/S_(B)), orL(1−S_(A)/S_(B)) nm.

Assuming the speed of the slowest traveling light, λ_(A), as areference, the λ_(B) light's phase shift relative to the λ_(A) light isthe number of the signal's wavelengths represented by distanceL(1−S_(A)/S_(B)). Since the λ_(B) light travels 2π radians in λ_(B) nm,the λ_(B) light's phase shift relative to the λ_(A) light isL(1−S_(A)/S_(B))(2π/λ_(B)) radians.

Signal dispersion may cause interference among sequential signal pulses,particularly where the optical fiber is relatively long. For example,assume that two pulses over the same wavelength range are transmittedover a fiber. If the fastest traveling light in the second pulseovertakes the slowest traveling light in the first pulse, the pulsesinterfere, and information may be lost.

Optical filters may be constructed to selectively modify phase toprevent such interference. A system may include one or more filters thathas a predetermined phase shift at each wavelength to counteract thedispersion caused by the fiber. Given that the phase shiftcharacteristics of a fiber optic cable may be determined as describedabove, a filter may be designed that substantially or totally reversesthe phase shift over the cable to reduce or eliminate phase shiftinterference. Such a filter may be designed to pass incident light atvarying speeds, depending on wavelength, in a manner opposite that ofthe fiber. Assuming a filter designed to correct dispersion in the fiberdescribed with respect to FIGS. 10A and 10B, for example, light atwavelength λ_(A) is passed through the filter at a first speed, andlight at wavelength λ_(B) is passed at a second speed that is slowerthan the first speed such that the λ_(B) light is again even with theλ_(A) light as it emerges from the filter. The speeds of the light atthe wavelengths between λ_(A) and λ_(B) are similarly set so that lightat all wavelengths leaves the filter even with the λ_(A) light, therebyreproducing the pulse illustrated in FIG. 10A.

An optical fiber system may include multiple filters intermittentlyalong a communications line. Referring to FIG. 10C, for example, filters92 may be disposed between adjacent optical fiber lengths 94 (forexample of a length L as described above with respect to FIGS. 10A and10B). Light from each length 94 is output to a collimating lens 96 thatdirects the light to its adjacent filter 92. A lens 98 focuses theemerging signal to the next optical fiber length 94.

Optical filters for correcting phase shift may be constructed usingtechniques similar to one or more of those discussed above, except thata desired phase shift spectrum, not necessarily a desired magnitudetransmission spectrum, is the guiding design criteria. An exemplaryoptical filter design procedure that accounts for phase shift isdiscussed at J. A. Dobrowolski and D. Lowe, “Optical Thin film SynthesisProgram Based on the Use of Fourier Transforms,” Applied Optics, Vol.17,No.19, p.3039 (Oct. 1, 1978).

The construction of any particular filter may depend on the length ofcable to which it is to be attached. Thus, filters might be constructedfor cables of predetermined lengths, for example 1 or 10 kilometers. Tocorrect dispersion in a particular system, as many filters as needed areintermittently installed over the length of cable. For example, iffilters are designed for 10 kilometer lengths of a certain type of fiberoptic cable, and the system includes a 95 kilometer length of thiscable, nine filters may be installed, each at a 10 kilometer interval.Although the remaining 5 kilometers may cause no significant distortion,a filter may be designed for this length as well.

Filter Modifiers and Erbium-Doped Fibers as Superfluorescent LightSource

Erbium doped fibers are sometimes used to amplify signals in fiber opticcommunications systems. As should be understood in this art, afluorescing erbium fiber emits radiation in a random pattern. If it isin line with a fiber optic cable, however, it emits two photons alongthe fiber optic path for each single photon received from the cable. Inthis manner, the erbium-doped fiber acts as a fiber optic amplifier, thegain of which is typically described in dB. Fibers may be insertedperiodically in long fiber optic cables, for example every 10kilometers, to achieve a desired gain.

Although erbium fibers are traditionally used as amplifiers, they mayalso be used as light sources in conjunction with optical filter systemsas described above. As described in more detail below, erbium fibers maybe used to create light signals having a relatively broad, yet limited,wavelength band favorable for the use of Dobrowolski-type opticalfilters. In addition, optical filters may be produced to modify thelight signal from the fibers to a standard predetermined form, thusallowing the use of standard, easily manufactured optical filters.

One problem with conventional fiber optic communication systems is theirlimited channel bandwidths. Since it is desirable to simultaneouslytransmit as many signals as possible over each channel, it is desirablein conventional systems to use signals, such as pulses, having as narrowbandwidths as possible. The pulses must be sufficiently separated inwavelength, however, to avoid crosstalk.

As described above, one solution to this problem is to use signalscomprising orthogonal pulses which overlap in wavelength. Because thesignals can overlap in wavelength, many more signals may besimultaneously transmitted over a single channel than in conventionalsystems. Because the signals are orthogonal, downstream optical filtersare able to compress the data to retrieve the information carried by thesignals.

In general, the orthogonal signals are created by passing light throughoptical filters that output to an optical fiber cable. Each filter'stransmission spectrum filters the light to a predetermined spectrum thatis orthogonal to the spectra created by the other filters. Pulses arecreated by pulsing the input light. Because the overlapping signals,which may also be referred to as “channels,” are orthogonal, eachchannel's pulses can be detected by a corresponding downstream filterwithout interference from the other channels.

There are an infinite number of ways to create a set of orthogonalfunctions. It may be desirable, however, to standardize these functionsto allow the use of standardized filter sets. A set of orthogonal sinewaves, for example, can comprise a relatively straightforward set offunctions.

The characteristics of light output from these filters, however, dependsupon the characteristics of the input light. An optical filter whosetransmission pattern defines a sine wave will output a sine wave signalonly if the input light has a constant intensity across the filter'swavelength range. If a sine wave output is desired from a non-constantintensity light source, the filter design may be altered to account forthe uneven input so that the filter nevertheless produces a sine waveoutput. In either approach, however, standard, interchangeable filtersrequire a standard light input.

Because the filters' output signal depends on the input light, andbecause input light differs depending on the light source, an orthogonalfilter set for use in systems having different light sources mustaccommodate the different light input if it is to output a standard setof signals in those systems. In one approach, an orthogonal filter setis created for each different light source. The output spectra ofvarious types of light sources, for example lamps or lasers, are easilymeasured. Filter sets are created for each source, each filter set beingconfigured to produce orthogonal functions for the particular lightpattern produced by its particular light source. Another configuration,however, employs a single filter set that assumes a standard lightsource spectrum. Modifying filters are made for each light source tomodify its output to the standard spectrum. One convenient standardlight source output may be a constant intensity (“flat”) signal.

The second approach has certain advantages over the first. Primarily, itpermits the use of standard filter sets for all optical communicationssystems. Light sources can be packaged with an appropriate modifyingfilter or filters so that these light source “packages” can be usedinterchangeably in any system.

The orthogonal signals of either filter approach discussed above havesome operative bandwidth. Because some light sources output light overwider or different bandwidth ranges than others, the choice of a givenset of orthogonal functions over a given bandwidth range may precludethe use of light sources which do not output light over that range.

Assuming that the desired standard input light signal is a flat signal,one possible light source is a laser that produces a very sharp, square,narrow bandwidth pulse. This has the advantage of providing flat inputlight signals to the orthogonal filter set without the use of modifyingfilters. As indicated above, flat input signals would allow the use oforthogonal filters with sine wave transmission patterns to produce sinewave orthogonal signals. Because the construction of Dobrowolski opticalfilters depends upon the Fourier transform of the orthogonaltransmission functions, construction of sine wave filters is relativelysimple. Thus, the use of flat pulses permits the use of easilyconstructed sine wave filters.

The size of a Dobrowolski filter, however, depends on the signalbandwidth. Because laser pulse bandwidths are so narrow, typically afraction of a nm, it is generally impractical to create Dobrowolskifilters for use with lasers. Even though the use of Fourier componentswould result in a relatively simple filter design, the filter itselfwould be very large.

Accordingly, at least when using filters constructed by the Dobrowolskimethod, it is desirable to use a broad band light to simplify filterconstruction. A lamp emits light covering hundreds of nm. Lamps are,however, inefficient. They require a relatively great deal of power,and, while they also output a great deal of power, it is distributedover the lamp's entire wavelength range. Thus, there is relativelylittle output power over the filters' operative bandwidth.

On the other hand, an excited erbium-doped fiber emits light over arelatively broad, yet limited, wavelength range (about 10 nm to 20 nm).Thus, the power output over its output bandwidth is greater than theoutput over a similar range from a lamp, yet the range is broad enoughto permit construction of Dobrowolski filters of an acceptable size.

Referring to FIG. 11, an erbium fiber 100 (or multiple fibers attachedend to end) is attached to a fiber optic cable 102 by an opticalconnector 104. An adjacent laser 106 excites the fiber 100 so that itemits its broader band light into the fiber optic cable. Although theexcited erbium fiber emits light in a random pattern, sufficient lightis input to the cable to create a suitable signal. The outputcharacteristics of erbium fibers should be understood by those ofordinary skill in the art, and the number of erbium fibers used may beselected according to the requirements of a given system. Furthermore,erbium fiber amplifiers may be used at suitable points to amplify thesignal if needed.

The output signal from erbium fiber 100 travels along optical fiber 102to a collimating lens 108 which directs the light to a modifying filter110. The modifying filter is, as described above, designed specificallyto modify the erbium fiber's output signal to a predetermined output. Ifa flat signal is desired, for example, the modifying filter may be aDobrowolski filter having a transmission pattern that passes 100% of thelowest intensity light within the erbium fiber's output spectrum andthat scales light at all other frequencies to that level. A lens 112focuses the flat signal to a fiber optic cable 114 for output to anelectrooptic modulation unit. Erbium fiber 100 and modifying filter 110may be packaged as a unit disposed in a housing 116, that may beinterchanged with other light source units.

The electrooptic modulation unit includes a collimating lens 118 thatreceives and collimates the signal from fiber optic cable 114 andoutputs the collimated light to a set of optical filters 120. Eachfilter 120 filters light received from lens 118 into a light signal thatis orthogonal to the signals from every other filter 120. A respectivelens 122 focuses the signal from its filter 120 to a fiber optic cable124 that, in turn, carries its signal to a modulator 126. Each modulator126 is controlled, for example by a computer or other suitableprocessing device, to intermittently pass the signal to a respectivefiber optic cable 128, thereby creating a series of pulses in such amanner that the number and/or frequency of pulses carries information.If the signal output by unit 116 is a standard shaped signal, thefilters 120 may be standard filters generating a set of standardorthogonal functions. A lens 130 receives the orthogonal pulses andfocuses them to an output fiber optic cable 132.

A series of downstream optical filters (not shown) receives the signalcarried by cable 132. One or more downstream filters filter the samefunction as a corresponding filter 120 in the electrooptic modulationunit. Thus, the downstream filter(s) detects the presence of each pulseemitted by the modulator 126 of the corresponding upstream filter 120.Since the pulses of the signal on line 132 are orthogonal to each other,each downstream filter detects pulses only from its correspondingupstream filter 120.

Although the upstream filter filters the same function as the downstreamfilter, the upstream and downstream filters need not be identicallyconstructed. For example, the upstream filter may comprise twophysically distinct filters that combine to generate a signal that isdetected by a single downstream filter having a transmission spectrumdifferent from that of either upstream filter.

In another embodiment, modifying filters and orthogonal filters areretrofit into existing WDM systems. The WDM systems may transmit aseries of individual pulses spaced apart over a particular wavelengthrange. Orthogonal filter pairs may be inserted into the WDM system todivide the WDM pulses themselves into orthogonal sub-components. Thiscreates a plurality of orthogonal channels within the existing WDMchannels.

An exemplary WDM retrofit system could be schematically illustrated bythe arrangement shown in FIG. 11 downstream from housing 116. Signalscreated by a conventional WDM source travel along cable 114 to acollimating lens 118 that outputs to filters 120. A group of filters 120is provided for each WDM channel, and a filter within each groupcorresponds to each orthogonal signal. Thus, assuming that there arefour WDM channels and that there are four orthogonal signals within eachchannel, sixteen filters 120 are used.

A bandpass filter may be placed upstream from each filter or filtergroup to limit the light received by each filter to its correspondingWDM channel. The four filters in each group receive light pulses onlywithin their channel's wavelength range, and each filters this lightinto a signal that is orthogonal to the signals from every other filterin its group.

Although not necessary, the same group of four transmission spectra maybe used in each of the other three filter groups. A lens 122 downstreamfrom each filter 120 focuses the filter's output signal to a fiber opticcable 124 that, in turn, carries its signal to a modulator 126. Whileseparate modulators are illustrated in FIG. 11, it should be understoodthat a single modulator may be used for all filters associated with asingle WDM channel. Each modulator may be configured to pulse theorthogonal signals within the period of the WDM pulse to code furtherinformation within each pulse.

Again, a series of downstream filters 120′ receives the signal carriedby cable 132. One or more downstream 120′ filters filter the samefunction over the same wavelength range as a corresponding filter 120.Thus, the downstream filter(s) detects the presence of each pulseemitted by the modulators 126. The downstream filters output to the WDMdetection system, which is now able to effectively detect sixteenorthogonal channels over four WDM channels.

Because the WDM pulses are not exactly squared, it may be desirable toinclude a modifying filter as described above to square each individualWDM pulse. A plurality of standard orthogonal filters, for example usingsine functions, can create orthogonal channels within each WDM channel,which can then be transmitted over a single optical line.

Data Metrics

Optical filters may also be employed to detect data reliabilityproblems. In general, an input light spectrum is unreliable when it issignificantly different from spectra the system normally processes. Inregression vector analysis, for example, samples analyzed by the systemshould be representative of the samples from which the regression vectorwas made. If they are not, conclusions drawn from the illuminatedsamples' light spectrum may be unreliable.

Reliability monitoring methods typically measure some form of distance,for example Euclidian distance, normalized Euclidean distance, orMahalanobis distance, between one or more sample spectra and arepresentative spectrum. To determine Euclidean distance, an averagespectrum is determined from a number of sample spectra known to bereliable. The distance between a measured spectrum and the resultingaverage spectrum is the square root of the squared difference betweenthe average spectrum and the measured spectrum at each wavelength.

One way to detect errors in optical filter systems as described hereinis to output the intensity of the sample spectrum to a computer whichperforms a suitable mathematical analysis. It is also possible, however,to estimate these calculations in a manner that can be effected by anoptical filter.

Assuming Euclidean distance, for example, between a sample spectrum andan average spectrum over three wavelengths x, y and z, the distance R isdescribed by the following equation:R=((x _(A) −x _(S))²+(y _(A) −y _(S))²+(z _(A) −z _(S))²)^(1/2)where x_(A), y_(A), z_(A) is the intensity of the average spectrum atwavelengths x, y and z and where x_(S), y_(S), z_(S) is the intensity ofthe sample spectrum at the same wavelengths. The Euclidean distance Rvaries inversely to the reliability of the sample spectrum. That is, thegreater the R value, the greater the distance between the samplespectrum and the average spectrum and the lesser the reliability of thesample spectrum.

Expanding the distance equation,R ² =x _(A) ²−2x _(A) x _(S) +x _(S) ² +y _(A) ²−2y _(A) y _(S) +y _(S)² +z _(A) ²−2z _(A) z _(S) +z _(S) ²,orR ²=(x _(A) ² +y _(A) ² +z _(A) ²)+(x _(S) ² +y _(S) ² +z _(S) ²)−2(X_(A) X _(S) +Y _(A) Y _(S) +Z _(a) Z _(S)).If the sample spectrum is similar to the average spectrum, the termx_(S) ²+y_(S) ²+z_(S) ² is approximately equal tox_(A)x_(S)+y_(A)y_(S)+z_(A)z_(S). Under this assumption,R ²=(x _(A) ² +y _(A) ² +z _(A) ²)−(x _(A) x _(S) +y _(A) y _(S) +z _(A)z _(S)).

The term (x_(A)x_(S)+y_(A)y_(S)+z_(A)z_(S)) is the dot product of thesample spectrum with the average spectrum. Because the intensity valuesx_(A), y_(A), and z_(A) are known, an optical transmission filter may beconstructed to transmit this pattern. For example, assuming “values” ofx_(A), y_(A), and z_(A) of 2, 3 and 5, respectively, the filter'stransmission spectrum could be 40%, 60% and 100% at respectivewavelengths x, y and z. If the exact magnitude of the average spectrumis desired, an amplifier can amplify the filter's output by a factor offive. If this “average spectrum” filter receives and filters the samplespectrum light x_(S), y_(S), z_(S), its output is equal to the dotproduct x_(A)x_(S)+y_(A)y_(S)+z_(A)z_(S).

FIG. 12 schematically illustrates an exemplary arrangement formonitoring sample spectrum reliability by Euclidean distance using theabove approximation. A sample 134 is illuminated by a light source 136so that light from the illuminated sample is received by a collimator138. A bandpass filter 140 receives the collimated light and passeslight within the average spectrum wavelength range to an averagespectrum filter 142.

Average spectrum 142 is an optical filter, for example a Dobrowolskitype filter, having a transmission spectrum proportional to the averagespectrum as explained above. The average spectrum may be determined bymeasuring the spectra of several samples that are known to be reliableand averaging the values of the spectra at each wavelength. A lightdetector 144 receives and measures the intensity of the output of filter142. Light detector 144 may be any suitable light detection device, asshould be understood in this art, capable of detecting light intensityover the average spectrum's wavelength range.

Detector 144 outputs an electrical signal to an op amp circuit 146 thatcompares the signal with a value provided by a sample and hold circuit148. If the magnitude of the difference between the signals fromdetector 144 and circuit 148 is more than a predetermined amount, op ampcircuit 146 outputs a signal to an output device 150, which may be anLED, a computer, or other suitable downstream processing or displaydevice.

The configuration of a suitable op amp circuit 146 should be understoodby those skilled in this art. Similarly, sample and hold circuit 148 cancomprise any suitable circuitry, for example including one or morepotentiometers, for providing a predetermined voltage level to the opamp circuit. The configuration and construction of these circuits arenot essential to the present invention in and of themselves and aretherefore not described in detail herein.

The output of sample and hold circuit 148 is preferably a signal equalor proportional to the output of detector 144 when sample 134 is theaverage sample. To determine this value, a second average spectrumfilter (not shown) filters light from a constant intensity light sourceso that it outputs the average spectrum. For example, assuming thetransmission rate of the average spectrum filter is 100% at thewavelength(s) having the highest intensity within the average spectrumand that the transmission rate at each other wavelength is equal to theintensity of the average spectrum at that wavelength as a percentage ofthe highest intensity, the intensity of light from the constantintensity light source across the filter's operative wavelength range isequal to the highest intensity within the average spectrum. The averagespectrum light output from this second average spectrum filter isdirected to the average spectrum filter 138 illustrated in FIG. 12.Thus, the output of detector 144 is proportional to or, for example ifthe detector's output is amplified by an appropriate gain as describedabove, equal to the dot product of the average spectrum with itself, orx_(A) ²+y_(A) ²+z_(A) ². Sample and hold circuit 148 is automatically ormanually set (as indicated at line 152) to this value.

Op amp circuit 14 subtracts the output of sample and hold circuit 148from the output of detector 144 when filter 142 receives light from asample 134 as shown in FIG. 12. Since the reliability measure R² is(x_(A) ²+y_(A) ²+z_(A) ²)−(x_(A)x_(S)+y_(A)y_(S)+z_(A)z_(S)) , thesubtraction of the circuit 148 output, x_(A) ²+y_(A) ²+z_(A) ², from thedetector 144 output, x_(A)x_(S)+y_(A)y_(S)+z_(A)z_(S), is equal to −R².

As discussed above, the output of op amp circuit 14 this signal is equalor proportional (depending on amplification of the filter 142 output inthe sample measurements and on the calibration of sample and holdcircuit 148) to −R². Thus, device 150 may be a computing device whichlogs and/or reports this value and/or which mathematically converts thevalue to the Euclidean distance R. In other embodiments, an acceptablevalue of R, and therefore −R², may be known, and op amp circuit 146 maybe configured to output a signal only when the difference between itsinput signals is beyond that value. In this case, device 150 may be anLED or audible alarm activated by the op amp circuit.

As noted above, the arrangement shown in FIG. 12 operates on theassumption that the sample spectrum is similar to the average spectrum.If an illuminated sample 134 is so different from reliable samples thatthe assumption does not hold, the output from detector 144 generallydiffers from the output of sample and hold circuit 148 by a relativelylarge amount, resulting in an output from op amp 146 indicatingunreliable data. That is, the greater the dissimilarity between samplespectra and the average spectrum, the faster that the −R² value tends torise, thereby indicating unreliable data.

The same assumption, that the deviation between the sample spectrum andthe average spectrum is small, may be used in other distancemeasurements, such as normalized Euclidean distance and Mahalanobisdistance, to remove squared sample spectrum terms, thereby allowing theapproximation of the distance measurement through dot product termseffected by optical filters. The Mahalanobis distance is calculated notjust using standard deviation normalization, but using the covariance ofthe wavelengths. If the samples are considered in principal componentspace rather than as individual wavelengths, the covariance of the spaceis lost, and it is necessary only to consider the variance of eachcomponent at the wavelength of interest. Specifically,M=Q−k′B,where M is the Mahalanobis distance, Q is twice the sum of the principalcomponent magnitudes; k′ is a proportionality constant, and B is equalto Σ_(i)x_(i)b_(i), where x_(i) is the intensity of the sample spectrumat wavelength i; b_(i) is Σ_(j)(l_(ij))/(e_(j)c_(j)), l_(j) is theloading of principal component j at the wavelength i; e_(j) is therelative eigenvalue of principal component j, and c_(j) is the varianceof principal component j. If the data has not been mean centered, thefirst principal component is the average spectrum. The first term istherefore similar to the term used in the scale-invariant Euclideanmetric. This metric is accomplished by calculating the termΣ_(j)(l_(ij))/(e_(j)c_(j)) at each wavelength i and preparing an opticalfilter with transmission scaled to that value. Two filters may be usedwhere the summation yields a negative. Other than this, the process issimilar to that discussed above regarding Euclidean distance.

The system illustrated in FIG. 12 may be used within a larger opticalfilter system. For example, a beam splitter may direct light from sample138 to a set of optical filters configured to monitor the input signalfor one or more characteristics of interest.

Shaping Filters

The use of bandpass filters to assure that only light within the opticalfilter's operative range is passed to a light detector is discussedabove. In a specific embodiment, however, each optical filter in thesystem is disposed on a substrate which includes the bandpass filter.This permits mass production of bandpass blanks upon which opticalfilters may be deposited.

The bandpass filter may be constructed from layers of inorganic oxidessuch as niobium oxide or silicon dioxide by an electron-beam evaporationor reactive magnetron sputtering process, as should be understood bythose of ordinary skill in this art. The structure may be designed by aniterative needle method as found in commercially-available programs suchas TF-CALC (Software Spectra, Inc.).

The oxide material, and therefore the bandpass filter, may be disposedon a flat, transparent, wafer-like substrate made, for example, ofsilica. The construction of silica type substrates is commonlyunderstood and is therefore not described herein. The substrate/bandpassfilter may then serve as a blank upon which an optical filter, forexample a Dobrowolski type filter, is disposed by similar techniques.Dobrowolski filters may be made from oxide type materials in accordancewith the Dobrowolski method noted above and may be constructed on thebandpass filter. Since the substrate is transparent, incident light maypass through the substrate and bandpass filter to the optical filter.

Spectrometer on a Chip

In another embodiment of the present invention, optical filters aredisposed on photodiode surfaces of a detector array. As should beunderstood in the art, photodetectors may be constructed from arrays ofdevices such as photodetectors or photodiodes. Very small (for exampleapproximately 25 micrometers wide) photodiodes may be defined as asemiconductor substrate by a suitable process such as lithographicfabrication so that the resulting chip contains thousands of devices.Referring to FIG. 13A, each photodiode 154 of a photodiode array iscomprised of a p-n junction having a certain capacitance that is chargedby incident light and is connected by an electrical lead on the chip toan output device 156, for example a data storage and/or processor devicesuch as a computer.

An optical filter 158 having approximately the same dimensions as aphotodiode is disposed on each photodiode 154. The optical filters maybe Dobrowolski type filters disposed on the photodiodes by a mask orsputtering process. A bandpass filter 160 is disposed on each filter 158to limit the incident light to the filters' operative wavelengthrange(s).

Each optical filter 154 is constructed to transmit an orthogonalcomponent of the incident light to the photodiode below. Typically, thefilters will be used to analyze light about which little is known. Thus,the filters 54 may compress the light data into an appropriate set oforthogonal functions, for example Fourier components. Computer 156performs the inverse transform of the components to reconstruct thesignal. Sets of photodiode/filter pairs, each embodying a set of Fourierfunctions, may be disposed about the chip.

If the light's principal components are known, each optical filter (oroptical filter pair where dual filters are used to account for positiveand negative principal component terms) may be a principal componentfilter. Thus, each photodiode immediately detects the magnitude of theprincipal component filtered by its optical filter and outputs thismagnitude to computer 156 that is in turn programmed to reproduce theincident light from this information. The computer may then perform anydesired spectral analysis.

FIG. 13A is provided as a functional example of a filter series. Since avery large number of filters will typically be disposed within adetector array, a CCD arrangement as illustrated in FIG. 13B may beappropriate. The output of each filter is connected to a transistordevice 162 which is selectively switched open or closed by a shiftregister 164. The shift register 164 sequentially switches eachtransistor to output the voltage across the photodiode to computer 156.Computer 156 is programmed to receive the sequential signals, whichcorrespond to identifiable photodiodes 154, and to reconstruct theincident light spectrum or spectra for further analysis.

Accordingly, the optical filters permit the construction of aspectrometer on a small, portable IC chip which can replace conventionalspectrometers in existing spectroscopy systems. These IC's enjoy asignal-to-noise advantage over conventional spectrometers, which maylose 70% to typically over 90% of incident light. Losses for the presentIC's are generally less than 50%.

ElectroOptic Filters

In another embodiment of the present invention, an optical filter isconstructed to have a changeable response. Referring to FIG. 14, opticalfilter 166 includes an optical filter 168 configured to selectivelyfilter light by wavelength over the filter's wavelength range, forexample to effect a regression vector. A filter layer 170 constructedfrom a non-linear optical material (for example lithium niobate or poledpolymer films) or other electrooptic materials such as liquid crystalsexhibits refractive indices over the wavelength range of filter 168 thatvary in the presence of an electric field. Layer 170 is sandwichedbetween conductive glass plates 172 and 174. A voltage generated by avoltage/signal source 176, which may include a voltage regulatorcircuit, applied across layer 170 at plates 172 and 174 changes therefractive indices of layer 170 from a first state to a second state.Since both filters 168 and 170 contribute to the overall transmissionspectrum of filter 166, the selective application of voltage 176 acrosslayer 170 changes filter 166 from one transmission spectrum to another.

Both optical filters 168 and 170 may be constructed using theDobrowolski iterative layering approach. The process begins with a basematerial having a particular refractive index. Subsequent layers areadded in an iterative procedure to achieve a desired transmissionspectrum. This is generally done by a computer program. If phase shiftis not a concern, there may be many, if not an infinite number of, waysto construct the layers to achieve a desired transmission pattern forthe filter.

Creating filter 166 is similar to solving simultaneous equations.Because there may be various ways to make each of the filters 168 and170, they are designed so that they cooperate to produce a first desiredtransmission spectrum when voltage 176 is applied across filter 170 andto produce a second desired transmission spectrum when the voltage isnot applied. In other words, the requirement that filters 168 and 170must combine to produce different predetermined transmission underdifferent conditions is a limitation in the layering design for eachfilter. This process should be relatively straightforward, for example,if filter 166 embodies a Fourier function, since changing the filter'soverall refractive index merely shifts from one Fourier function toanother.

A filter 166 might be designed to effect a certain regression vectorwith the voltage across filter 170 is disconnected but to effect thenegative of the regression vector with the voltage applied. Thus, lightfrom an illuminated sample may be passed through filter 166 with voltage176 disconnected so that the filter performs the dot product of theincoming light and the regression vector. When voltage 176 is thenapplied across filter 170, however, filter 166 applies the negative ofthe regression vector. If measurements of light intensity under thesetwo conditions are subtracted, the result is proportional to theproperty to which the regression vector corresponds. This is aconvenient way to remove any DC component that may otherwise arise froma variety of causes and that may interfere with accurate measurement.

This configuration also allows construction of a single filter toanalyze a sample for multiple properties by selectively applying one ormore voltages to particular filter layers. For example, the filter mayinclude several non-linear optical material layers 170 to produce asingle filter 166 that can apply multiple functions. The number ofdesired functions increases the complexity of filter design.

For layer 170 to exhibit the desired non-linear electroopticcharacteristic, molecular elements of the material comprising the layermust be aligned to facilitate application of the electric field. Thismay be achieved by a number of methods of producing non-linear opticalmaterial wafers, for example poling or brushing a polymer layer, asshould be understood by those of ordinary skill in the art.

In another method, however, a polymer layer is deposited on plate 174,for example by a vaporization technique. Light polarized 90° to thedesired direction is applied to the polymer layer to remove molecularelements of undesirable polarization. The light quickly excites andburns away those elements aligned with its polarization. Continuedapplication of the light burns away elements increasingly angularlyoffset from the light's polarization until elements approaching a 90°offset are removed. The light does not excite the elements disposed atthe 90° angle, and those elements will remain in tact within the layer.The duration of the light's application to the polymer layer determinesthe angle at which elements not exactly aligned in the desirabledirection are retained.

Color Separation Filters

Color videography or photography systems typically use three colorchannels: red, green and blue. In this embodiment of the presentinvention, optical filters may be used in a color system to detectpredetermined characteristics of a sample and to output color signalsindicative of these characteristics.

In one exemplary embodiment of the present invention illustrated in FIG.15, a light source 178 illuminates a sample 180. The sample may beilluminated by any suitable method, for example by reflection of ambientsunlight. A video system camera lens 182 within a housing 184 directslight from sample 180 to a bandpass filter 186 configured to pass lightwithin the operative wavelength range of downstream optical filters 190.A beam splitter 192 separates the light into three light beams, eachdirected to a respective filter 190.

Each filter 190 directs its output to a detector 194, for example a CCDcamera, that is associated with a particular color (for instance red,green or blue). The construction of CCDs and their use within videosystems should be understood and is, therefore, not described in detailherein. Briefly, however, each CCD 194 may include an array ofphotodiodes, each corresponding to a pixel in an electronic image.Because there are three CCDs (one for each of the three colors that maybe present in any pixel) in the system illustrated in FIG. 15, there arethree photodiodes for each pixel. One or more shift registerssimultaneously scan the three CCDs to output the voltage stored at thecorresponding photodiode on each CCD to a display device 196. Displaydevice 196, for example a video monitor, receives the three signals anddisplays at the appropriate pixel a color that is a combination of red,green and blue at intensities determined by the respective signalintensities. The system repeatedly scans for each three-photodiode groupthe CCD's.

The construction and operation of video systems and their componentsshould be well understood. In conventional systems, the filters upstreamfrom the color filters CCDs pass only the light color with which theirdownstream CCD is associated. Thus, the light incident on the CCD is theportion of the image received by the video lens consisting of thatcolor. Each photodiode is charged to a level indicative of that color inthe image at the photodiode's position. By scanning the photodiodes andcombining the information from each three-photodiode group, the systemis able to display the image at the display device.

In the video system illustrated in FIG. 15, however, each filter 190 isan optical filter, for example a Dobrowolski type filter, that filters afunction, for example a regression vector, that is associated with aparticular characteristic of the sample. The light output from eachfilter is therefore proportional to the strength of the characteristicdetected in the light of the sample's image rather than the intensity ofa color. Accordingly, the output of any CCD photodiode indicates thedegree to which the characteristic with which its filter 190 isassociated is present at that area of the image. Display device 196therefore displays the image in a color scheme that depends upon therelative strength of each characteristic identified by the respectivefilters 190 at each pixel.

For example, assume the device is contained within a small hand-heldhousing 184, that the sample 180 is the surface of an aircraft, and thatthere are three primary characteristics of the surface that contributeto its structural weakness. Filters 190 may be designed as describedabove to effect a regression vector for light reflected from the surfaceto identify the presence of each characteristic. When the device ispassed over the surface so that light is reflected from the surface tolens 182, the surface's image displayed by device 196 is comprised of acombination of red, green and blue at each pixel indicative of thepresence of each characteristic at that position in the image. A colorscale may be developed to which the display output may be visually orautomatically compared to identify those areas of the aircraft surfacein which the three characteristics exist in such proportion to indicatea serious weakness.

It should be understood that FIG. 15 is provided for illustrativepurposes only and that the device may be constructed in various suitablemanners. For example, the components 182, 186, 192, 190, 194 and 196 maybe constructed as a unitary or nonunitary device. Beam splitter 190 maybe replaced by other suitable components or arrangements and may beomitted, for example where the filters are all disposed to receive lightfrom filter 186. Furthermore, while three filters respectivelyassociated with three colors are provided to illustrate how the presentinvention may be utilized within a conventional video system anysuitable number of filters and colors may be employed. For example,where the optical filters effect regression vectors that includepositive and negative components, two CCDs may be provided for eachcolor, one CCD receiving the output of a positive component filter andthe other receiving the output of a negative component filter.

Artificial Neural Network Stages

Neural networks include a series of nonlinear steps each having one ormore processing elements. Each processing element may include its ownmemory and processing capacity so that it performs a predeterminedoperation upon the information it receives. Each processing elementreceives one or more inputs and produces a single output which may bedirected to as many downstream processing elements as desired.Processing element output signals are transmitted over unidirectionalsignal channels.

The architecture and component structure of the various neural networktypes should be well understood and are not discussed in detail herein.In an embodiment of the present invention, however, optical filters areused to modify data that is input to a neural network processingelement. Referring to FIG. 16, a neural network 198 includes an array ofprocessing elements 200. Two processing element stages A and B eachinclude four processing elements 200.

The input to each processing element in stage A is an optical filterarrangement 202. Each arrangement 202 includes a collimating lens 204receiving light from a sample 206 illuminated by a light source 208 anddirecting the collimated light to a bandpass filter 210. Bandpass filter210 limits the light to the wavelength range of an optical filter 212.Optical filter 212 filters light from bandpass filter 210 and outputsthe filtered light to a detector 214 which outputs a signal to itscorresponding processing element 200.

Accordingly, the output of each optical filter 212 is an input for aprocessing element 200. Each optical filter may be, for example, aDobrowolski type filter configured to effect a desired function relatedto one or more characteristics of the sample 206 that are to be analyzedby network 198. In FIG. 16, for example, each optical filter 212 mayeffect a regression vector to identify the presence or degree of acertain characteristic of sample 206 through the light from the sampleor may embody a principal component or other desired function. Thisinformation then becomes data for the network's stage A. It should beunderstood that the optical filter arrangements are illustrateddiagrammatically and that for ease of explanation only a single opticalfilter 212 is included with each arrangement. It should be understood,however, that more are possible. For example, where a regression vectorincludes positive and negative component, a dual optical filterarrangement may be employed as discussed above with respect to FIG. 3B.

Hybrid Chemical—Orthogonal Methods

Substances are sometimes contaminated with other substances that are notreadily apparent to visual, chemical or spectroscopic analysis. Byaddition of a certain reagent, however, a new compound is formed thathas a larger spectroscopic signature and that is, therefore, more easilydetected. For example, the chemical TCE (trichloroethylene) is anenvironmental hazard that may be almost undetectable when present inwater. In the Fujiwara process, for example, a reagent added toTCE-contaminated water reacts with TCE so that the presence of thecontaminant is detectable.

Unfortunately, the Fujiwara reagent may react with other substances toproduce a very similar color, making visual identification difficult.However, even where the different reactions produce similar colors andspectroscopic signatures, there are differences, and an optical filteraccording to the present invention may embody a regression vector toidentify a particular chemical produced only by the reaction with thesearched-for contaminant, thereby eliminating the need for visualinspection and the possible resulting uncertainty. A sample may beanalyzed, for example, in a system such as illustrated in FIG. 3A or 3B.

Designer Fluorophores

In DNA systems, fluorescent “tags” may be attached to DNA bases by a DNAsequencer. The DNA is cleared, one base at a time, and the bases areoutput in a fluid stream through which light is passed to aspectroscope. The resulting spectrum is analyzed to determine which tagsare present, thereby identifying the DNA bases.

There are significant limitations with this approach. Primarily, it isrelatively imprecise, unless the tags are very different from oneanother, and tags are therefore typically selected from fluorophoreshaving widely differing spectral signatures. Since each tag must besignificantly different from all other tags, the number of availabletags is limited. Although the method is sometimes acceptable for DNA,which has only four bases, the limited number of significantly differenttags poses a problem regarding proteins, which have twenty-three aminoacids.

Optical filters in accordance with the present invention, however, mayapply regression vectors to distinguish fluorescent tags, even where thetags are more similar than permissible under conventional methods. Forexample, referring to FIG. 17, a DNA sequencer 216 applies fluorescenttags to DNA bases and outputs the tagged bases to a fluid stream througha fluid conduit 218. A laser 220 excites the stream through a microscope222 as it passes through the conduit. Laser 220 may be, for example, anargon or ultraviolet device. A collimator 226 receives the light fromthe illuminated fluid and directs the light to a series of opticalfilters 228, each of which effects a regression vector to identify aparticular fluorophore tag. It should be understood that a dual filterconfiguration as discussed above with respect to FIGS. 3A and 3B may beemployed where the regression vector includes positive and negativecomponents.

Although three filters are illustrated, it should be understood that anynumber may be used, depending on the number of fluorophore tags thesystem employs. Bandpass filters 230 limit the light to the operativewavelength range of the filters 228. Each detector 232 outputs anelectrical signal that corresponds to the intensity of the light signalreceived from a corresponding filter 228 to, for example, a computer. Bymonitoring the detector outputs, the computer is able to identifyspecific fluorophore tags.

Autocorrelation Function and Normalization

In one signal normalization method, a signal's magnitude is measured inany suitable and consistent manner (for example at a peak or asintegrated over a given wavelength range) to proportionally scale theamplification level of an amplifier. For example, assume a signal isinput to an amplifier having a ten times gain that is inversely scaledby the value of a signal input to the amplifier. A detector is placedupstream from the amplifier and is configured to measure the signal'smagnitude and to output a corresponding electrical signal to theamplifier, or to a circuit controlling the amplifier, so that theamplifier gain is divided by the signal magnitude. Thus, if the incomingsignal has a magnitude of 5, the amplifier gain of 10 is divided by 5 toa gain of 2. If the incoming signal has a magnitude of 10, the amplifiergain is 1. Thus, within the operating range of the amplifier, themagnitude of the amplified signal is always 10.

A system for normalizing an optical filter signal is illustrated in FIG.18. A light source 234 illuminates a sample 236. A collimator 238collimates the light and outputs to an optical filter 240, which effectsa desired function such as a regression vector. A detector 242 receivesthe light from the filter and outputs an electrical signal 244 thatcorresponds to the intensity of the light it detects. Where theregression vector has positive and negative components, a dual filterstructure as discussed above with respect to FIGS. 3A and 3B may beused. Signal 244 of FIG. 18 would be output by processor 58 of FIG. 3B.

An amplifier 246 amplifies signal 244, outputting the amplified signalto a computer 248. Amplifier 246 is a variable amplifier having a gaincontrolled by an input signal 250. Amplifier 246 may be any wellunderstood device, and its particular configuration does not form anessential part of the present invention in and of itself. It may includeany associated control circuitry for receiving signal 250 anddetermining its gain responsively thereto. In this embodiment, theamplifier's gain is understood to be divided by the magnitude of signal250.

In FIG. 18, signal 250 is equal to signal 244. Thus, amplifier 246 isscaled by the instantaneous magnitude of the signal output by thedetector. It should be understood, however, that other scaling factorsmay be used. For example, an integrating circuit may be disposed alongthe path of signal 250 so that the amplifier gain is scaled by theaverage detector output over a predetermined time period. Further, whilecomputer 248 is illustrated as the output device, this is for exemplarypurposes only, and the amplifier output may be directed to any suitabledownstream processing device, depending on the function of theparticular system.

Normalization may be employed to perform an autocorrelation of the inputsignal, specifically the dot product of the input signal with the inputsignal average, to indicate the input signal's reliability. The inputsignal average is predetermined. Where a sufficient number of samples ofknown reliability are available, the spectrum of each sample may bemeasured and averaged by wavelength to determine an average spectrum.

If the input signal is equal to the average signal, the dot product ofthe two is a known value. As the input signal differs from the average,however, the dot product changes from this known value. The magnitude ofthe change is a measure of the signal's reliability.

In spectroscopy systems, the overall intensity of the input signalgenerally depends upon the intensity of the light source used toilluminate the sample. Information is carried by the spectral shape ofthe light from the sample. Since light source intensity tends to varyover time, it is desirable to normalize the input signal in performingthe dot product described above. Once the sample signal is normalized,and if the light used to illuminate the samples to determine the averagespectrum is of a consistent magnitude or if the average spectra arethemselves normalized before determining the sample spectrum, deviationin the dot product is due to differences between the spectral shapes ofinstantaneous input signals, not to differences in overall signalstrength. Accordingly, the dot product is preferably performed betweenthe normalized instantaneous signal and the average spectrum.

In terms of signal vectors, assume that S_(IN) is the normalizedinstantaneous input light signal, considered as a vector in wavelengthspace, that S_(I) is the unnormalized instantaneous input light signalvector, and that S_(A) is the average input light signal. The dotproduct of the normalized instantaneous signal and the average signalis:S _(IN) ·S _(A)=(S _(I) /n.f.)·S _(A)=(S _(I) ·S _(A))/n.f.,where n.f. is a normalization factor.

A system for performing this function, and thereby monitoring signalreliability, is illustrated in FIG. 19. The instantaneous input signalis output from collimator 238 to a beam splitter 252, which directs thesignal to an optical filter 254 and to a detector 242A. Although thebeam splitter reduces the overall intensity of the two divided signalsfrom that of the input signal, the ratio of the overall intensities ofthe divided signals is typically unimportant as long as it remainsconstant, and the output of detector 242A may be suitably used as anormalization factor. Under this condition, the signal output from thebeam splitter 252 to filter 254 may be considered S_(I) in the aboveequation.

Filter 254 is an optical filter having a variable transmission spectrumpatterned after the shape of the average input signal spectrum S_(A).This pattern is determined by recording and averaging the spectra ofseveral samples as described above. To optimize the signal-to-noiseratio, the transmission rate of filter 254 at the wavelength(s)corresponding to the highest intensity levels in the average spectrum is100%. The transmission rates at each other wavelength is equal to theratio of the average-spectrum intensity at that wavelength to thehighest average-spectrum intensity.

The output of filter 254 is equal to the dot product of the unnormalizedinstantaneous input light signal vector and the average input lightsignal vector, or (S_(I)·S_(A)). To divide by the normalization factor,variable amplifier 246, which amplifies the (S_(I)·S_(A)) signalreceived by detector 242B, is scaled by the output of detector 242A. Asdiscussed above, the normalization factor may be configured to anydesired form, for example by an integrator circuit.

Computer 248, or other suitable monitoring device, receives the outputfrom amplifier 246 to determine the difference between it and the dotproduct of the average spectrum with itself. As set forth above, thisdifference is an indication of input signal quality.

The system illustrated in FIG. 19 may be used within a larger opticalfilter system. For example, beam splitter 252 may also direct the inputsignal to a set of optical filters configured to monitor the inputsignal for one or more characteristics of interest. Thus, computer 248monitors data quality in real time.

Filter Simplification

Optical filters made by the Dobrowolski method are based on the Fouriertransform of the filter function. Accordingly, filter design andconstruction may be complex where the filter function's Fouriertransform contains high-frequency components. Filter design may besimplified, however, if the filter function is separated into segments.An appropriate offset function is added to the segments so that if afilter is constructed for each function segment, subtraction of thesegment filters' output is equal to the output of a single filtereffecting the original filter function. Segment filter construction maybe simplified where the offset function is chosen to minimize the effectof high frequency Fourier components of the segment functions.

One convenient manner in which to divide the filter function intosegments is by positive and negative values. Referring to FIGS. 20A, 20Band 20C, assume that an exemplary filter function A shown in FIG. 20A isdescribed as a transmission spectrum so that the greatest magnitudepoint on the graph is positive or negative 100%, depending on whetherthis point lies on the positive or negative portion. Function A isseparated into its positive segment B and its negative segment C inFIGS. 20B and 20C, respectively. Segment C is inversed so that B−C=A.Thus, if optical filters are constructed to embody B and C, their outputmay be subtracted to achieve the same output as a filter embodying thefunction A.

If any arbitrary function D (not shown) is added to both B and C, theresult is the same. The new functions may be described as B+D and C+D.Their subtraction, (B+D)−(C+D), is still equal to B−C, which is equal toA as described above.

As set forth above, however, functions B and C represent transmissionrate patterns with a maximum transmission rate of 100%. There can be noaddition to either segment by function D at those wavelengths where thetransmission rate is already 100%, and only limited addition where thetransmission rate is near 100%. The conflict may be resolved by scalingfunction A (and therefore functions B and C) so that the maximumtransmission rate is less than 100%, by not adding or partially addingfunction D at those wavelengths where functions B and C are at or near100%, by defining function D so that it causes no addition to eitherfunction over 100% at any wavelength, or by some combination of theseapproaches. Thus, while the purpose of function D described below is toreduce the complexity of the segment filters, it is also desirable todefine and/or implement the function to preserve the integrity of thesegment functions.

Function D is defined from analysis of the Fourier transforms ofsegments B and C, representations of which are provided in FIGS. 20D and20E, respectively. These graphs are for exemplary purposes only and arenot intended to represent the actual Fourier transforms of the functionsshown in FIGS. 20B and 20C.

Referring to FIGS. 20D and 20E, a frequency E is chosen above which thetwo transforms will be minimized. Minimization of the high frequencycomponents reduces filter complexity, and the lower the frequency atwhich frequency E is selected, the greater the filters are simplified.The magnitude of function D increases, however, as frequency Edecreases. Thus, frequency E is chosen to be as low as possible whilekeeping any effect of function D on data quality as described abovewithin suitable limits. Such limits will depend on the particularcircumstances of a given design.

The high frequency Fourier components are minimized by minimizing the“power” of the combined transforms. For example, at frequency x₁, thepower of the combined transforms is (−3)²+1²=10. The power at frequencyX₂ is 4²+(−2)²=20. Obviously, the power at each frequency is minimizedif the values of both the 20D and 20E graphs at those frequencies arezero. This is impossible, however, because the same function D must beadded to both segment B and segment C. That is, while a value of 3 mustbe added to bring the B transform to zero at frequency X₁, that valuewould also be added to the C transform at frequency X₁, leaving a valueof 4 and an overall power of 16. Because the effect of the highfrequency transform components is directly related to the combinedtransform power, this actually increases filter complexity.

Power is minimized, however, where the transform values are averaged ateach frequency and where the average is subtracted from both transforms.At frequency X₁, for example, the average value is (−3+1)/2=−1. Whenthis value is subtracted from both the B transform and the C transform,the transforms have a value of −2 and 2, respectively, resulting in apower of 4. Repeating this process at frequency X₂ provides a power of10.

Accordingly, the Fourier transform of function D at each frequency isdetermined by averaging the value of the B transform with the value ofthe C transform at that frequency, for all frequencies above E, andtaking the inverse of the result. Inverse transforming this frequencydomain function produces the wavelength domain function D. If D is addedto both the B and C segments, the resulting segment filters are lesscomplex in design and construction than filters designed from segmentfunctions B and C alone, while providing an equal or approximately equalcombined response.

Functions B and C illustrated in FIGS. 20B and 20 c are positive at allwavelengths. Function D might have negative components. It is thereforepossible that the modified functions B and C might have negativecomponents. This may be accommodated by adding a positive constant valueto function D at all wavelengths so that functions B and C are positiveat all wavelengths. Because the added constant has no frequency, it addsno complexity to filter construction. It does, however, add to functionD's magnitude.

Simplification methods utilizing DC offsets may also be applied to thedesign of Dobrowolski type filters embodying regression vectors havingpositive and negative components. Initially, a transmission spectrum isdetermined from the regression vector so that the entire transmissionspectrum is at or between −50% and 50%. The transmission rate at thewavelength(s) having the greatest magnitude may, for example, be set topositive or negative 50%, depending on the sign (positive or negative)of the intensity at each of these wavelengths. The transmission rate ateach other wavelength is 50% scaled by the ratio of the intensity atthat wavelength (positive or negative) to the magnitude of the greatestintensity. Accordingly, the transmission rate at these wavelengths isbetween −50% and 50%.

To determine the filter's transmission spectrum, the regression vectortransmission spectrum is increased by 50% at each wavelength. Thetransmission rates of this “modified” transmission spectrum, therefore,now range from 0 to 100%. Referring to FIG. 21, a Dobrowolski typeoptical filter 256 effecting the modified spectrum is disposed tooperate at a slight angle, for example 10°, with respect to the path ofincident light 258 from a sample (not shown). A first light detector 260is disposed beyond filter 256 to receive the light transmitted thereby.Because filter 256 is not perpendicular to the path of light 258, thefilter's transmission spectrum shifts by a certain predictablewavelength distance as described below. This shift is accounted for inthe design of filter 256 SO that the filter effects the transmissionspectrum that is desired if light 256 were normal to filter 256.

Light not transmitted is reflected at an angle of 20° from the path oflight 258. Detector 262 is disposed to receive this light. Each ofdetectors 260 and 262 outputs a signal corresponding to the intensity ofits incident light to a processor 264, which may be any suitable deviceor arrangement (such as a computer or circuitry) capable of performingthe appropriate mathematical functions. Processor 264 subtracts theoutput of detector 262 from the output of detector 260, halves thedifference, and outputs a signal equal to this value at 266.

It should be understood that various suitable filter and detectordispositions may be used. For example, rather than designing the filterfor operation at a relatively small angle such as 20°, the filter may bedesigned to operate at a larger angle, such as 45°. Prisms may be usedto bring the reflection and transmission side-by-side to a detectorpair.

Signal 266 is the signal that would have been output by a light detectorreceiving light transmitted by an optical filter effecting the originalregression vector transmission spectrum had that filter received light582. Thus, signal 266 is proportional to the dot product of the spectrumof light 258 and the regression vector. To illustrate, assume that R isthe original regression vector transmission spectrum (extending from−50% to 50%), that MR is the modified transmission spectrum (offset by50% and thereby extending from 0 to 100%), and that o is the offsetspectrum (positive 50% at each wavelength). Thus,MR=R+o, orMR=R+50%.If RF is a transmission spectrum describing light reflected from filter256,RF=100%(at each wavelength)−MR, orRF=100%−R−50%, orRF=50%−R.The output 266 of processor 264 is, therefore,½(MR−RF)=½(R+50%−50%+R)=R.

A single filter arrangement as in FIG. 21 may be used in place of thedual filter arrangement as illustrated in FIG. 3B. Further, it should beunderstood that either a single or a dual filter arrangement, asdesired, may be used to effect a vector having positive and negativecomponents as appropriate. Thus, while a single filter associated with asingle light detector may be illustrated in various embodiments hereinfor ease of illustration, it should be understood that eitherarrangement may be used where needed to accommodate transmission spectrahaving positive and negative components.

Temperature Correction for Optical Devices

Certain optical filters in accordance with the present invention may beconstructed by layers of interference films.

Materials, for example certain plastics and transparent oxides, used tomake these films have a nonnegligible coefficient of thermal expansion.Specifically, the refractive indices and linear dimensions of thematerials may change with temperature change. This may cause thefilter's transmission spectrum to shift to longer or shorterwavelengths. Generally, wavelength shifts upward as temperatureincreases. For example, a certain filter may be designed to have a 70%transmission rate at 900 nm. A given temperature change, however, maycause a wavelength shift so that the 70% transmission rate shifts to 895nm, with all other transmission rates experiencing a similar shift.

To counter this effect, the filter may be rotated about an axisperpendicular to the light's direction of travel, thereby changing theangle at which the light hits the filter surface. This also causes awavelength shift, either to longer or shorter wavelengths. The rotationis correlated with the temperature change so that the wavelength shiftcaused by one counteracts the wavelength shift caused by the other,leaving the filter's resulting transmission spectrum substantiallyconstant.

Once the wavelength shift caused by the filter's thermal expansion isknown, the angle of rotation θ needed to counteract the shift can bedetermined by the following equation:λ_(θ)=(λ₀ /n) (n ²−sin² θ)^(1/2),where λ₀ is the wavelength at which a given transmission rate appearsbefore expansion, λ_(θ) is the wavelength to which this transmissionrate shifts due to the expansion, and n is the effective refractiveindex of the filter materials. Since the filter is comprised ofmaterials having different refractive indices, the effective index ofthe coating stack is used.

In one preferred embodiment illustrated in FIG. 22, an optical filter268 is mounted on a stage, for example a frame 270, so that it isrotatable about an axis 272 perpendicular to the incident light 274. Atemperature-sensitive member 276 made of a material that expands andcontracts with temperature is attached at one end to the filter and atthe other end to a support 278 of frame 270. The expansion andcontraction of the element with temperature change rotates the filter onthe frame about the axis. The material and dimensions of the element,and its place of attachment to the filter relative to the axis,determine how much the element rotates the filter about the axis. Theseparameters may be determined by trial and error with a given filterconstruction or may be predicted if the thermal expansioncharacteristics of the filter material and of the temperature sensitivematerial are known.

Spectrum-corrected Light Source

In spectroscopy systems, the spectrum of light from sources such aslamps typically varies with the source's age. When the lamp is new,light from all parts of its filament is similar, resulting in arelatively uniform spectrum. Filaments, however, tend to develop thinspots. Under a constant voltage, current through the filament decreasesdue to the thin spots' higher electrical resistance. The reduced currentcauses the non-affected parts of the filament to cool slightly, therebycausing the light from these parts to shift slightly red. The thinspots, on the other hand, become hotter, resulting in a slight blueshift for this light. Thus, the overall spectrum experiences a modestbulge at either end, the majority of the bulge occurring toward theblue. Because information is carried by the spectral shape of light froman illuminated sample, such a change in the light source's spectrum mayaffect a spectroscopy system's accuracy. The degree to which this is aproblem depends on the system's sensitivity and the application forwhich it is used.

Rather than addressing the change in spectral shape, conventionalsystems generally focus on output power, which typically decreases in aworn filament as current decreases. To maintain an average lightintensity, such systems may employ one or more light or heat detectorsproximate the lamp and adjust the filament voltage responsively to thedetector. The output of the detector may be directed to a controlmechanism that controls the voltage applied to the filament so that asintensity decreases with the development of thin spots, the voltage isincreased to offset the power output loss. By addressing only lightintensity, these systems fail to account for spectral shape changes thatmay impact data integrity.

In contrast, the present invention addresses spectral change. Referringto FIG. 23, a collimator 284 directs light from a light source 280 to abandpass filter 286 that limits the light to the wavelength range of anoptical filter 288. Optical filter 288 has a transmission spectrumeffecting a regression vector that identifies change in the spectrum oflight source 280. The regression vector may be based on wavelengthbands, principal components or other orthogonal components, as describedabove regarding regression vector analysis. To determine the vector,spectra are measured from several new lamps at the voltage at which theyare intended to operate, and these measurements are averaged. Voltage isthen varied about the intended voltage (for example +/−1 or 2 volts).The regression vector is determined by calculating the distance betweenspectra within this range and the average spectrum and determining thecomponent coefficients as described above. In another method, samplespectra taken from several lamps over an extended period as the lampsage are compared with the average lamp spectrum.

A detector 290 outputs the intensity of the transmitted light to aprocessor 292 that determines from this information whether the lightsource spectrum is approximately what it is expected to be. That is, itdetermines whether or not the output of optical filter 288 issufficiently close to the output that would be expected if lamp 280 wereto emit the average spectrum. If not, the processor adjusts the powerinput to light source 280 until the detector 290 output indicates thatthe spectrum is again acceptable.

If the regression vector includes negative components, it may comprise adual filter arrangement as discussed above with respect to FIG. 3B or areflection detector arrangement as discussed with respect to FIG. 22.Since light from light source 280 may be employed in a spectroscopysystem to analyze a sample, light may be directed to both the sample andto collimator 284 by a beam splitter or other suitable arrangement.Thus, the feedback arrangement illustrated in FIG. 23 may approximatelymaintain the light source spectrum to maintain the integrity of theanalysis performed by that equipment.

Gratings

Optical filters may also comprise gratings designed to compress anincident light signal into a desired function. Optical gratings may beconstructed in various well-understood manners. For example, a top viewof a volume holographic grating 294 is provided in FIG. 24A. The gratingcomprises a polymer or gelatin layer whose density varies in apredetermined pattern from higher density regions 296 to lower densityregions 298. The density modulation determines the angle at which lightat a given frequency is reflected from the grating's surface. Since thereflection angle varies with frequency, reflected light is separatedinto its spectrum.

FIGS. 24B and 24C illustrate side views of two types of groovedreflection gratings, which may be constructed of various suitablematerials including metals and polymers. The grooves in the grating 294of FIG. 24B define a substantially sinusoidal shape and can be formedfrom a conventional photographic process. The distance A between thegrooves' peaks and valleys is the modulation depth, typically less thanor equal to 3 or 4 micrometers. The distance B between groove peaks maybe used to calculate the groove density (modulation density) C, which istypically within the range of 300 grooves/mm to 4800 grooves/mm. Thelight's angle of incidence and angle of reflection are indicated at αand β, respectively.

If the angle of incidence is constant, the angle of reflection dependsupon the modulation density C. The intensity of the reflected lightdepends upon modulation depth A. Referring to density in terms ofdistance B, λ/B=sin α=sin β, where λ is the wavelength of the incidentlight. Thus, assuming that the incident light is polarized (i.e. that αis constant) and that the grating has a uniform density (i.e., that B isconstant), the angle of reflection β varies with wavelength λ, and thereflected light is separated into its spectrum.

The grooves of the grating 294 of FIG. 24C define a sawtooth shape whichmay be formed by a mechanical or laser cutting tool. In this grating,modulation density again affects the reflection angle, but the pitch (or“blaze angle”) D is also a factor. If the groove pattern is uniform overthe groove, the angle of reflection of polarized light again varies withwavelength, and the grating separates the reflected light into itsspectrum.

When a reflection grating is used to separate light into its spectrum,the reflected light is typically directed to a light detector thatdetects light intensity regardless of the angle at which the light hitsthe detector. Thus, if a beam of polarized light is directed to agrating and reflected to such a detector, the light intensity at eachwavelength can be measured by measuring the intensity of light at thearea of the detector corresponding to that wavelength.

Referring to FIG. 24D, however, a target light detector 300 detectslight at only one angle of incidence. The detector detects no incominglight at other angles. A grating 294 includes three regions G₁, G₂ andG₃ having grooves separated by different modulation distances B₁, B₂ andB₃, respectively. Detector 300 is disposed with respect to grating 294so that light 302 reflected from grating 294 at an angle β is receivedby detector 300 at its operative angle of incidence. Thus, the detectormeasures the intensity of light 302. Detector 300 may be constructedfrom a lens or mirror that focuses parallel light 302 to a conventionallight detector.

Because the modulation density of grating 294 varies among regions G₁,G₂ and G₃, the regions reflect light at angle β having differentwavelengths λ₁, λ₂ and λ₃, respectively. Accordingly, if polarized lightis directed to the entire surface of grating 294, target area T₁ ofdetector 300 receives only light of wavelength λ₁, target area T₂receives only light of wavelength λ₂, and target area T₃ receives lightonly of wavelength λ₃. A mirror may be disposed between grating 294 anddetector 300. Since light 302 has a known angle of incidence to themirror and, therefore, a known angle of reflectance, the detector may bedisposed with respect to the mirror to receive and detect the reflectedlight 302.

By varying the areas of regions having certain modulation densities, anoptical filter 294 as in FIG. 24D may be configured to compress lightdata as do the Dobrowolski filters described herein. For example, assumea Dobrowolski filter transmits 50% of light of wavelength λ₁, 100% oflight of wavelength λ₂ and 25% of light at wavelength λ₃. A grating 294as in FIG. 24D may effect a function having the same spectral shapewhere area G₁ is half of area G₂, and area G₃ is one-fourth of area G₂.The output 304 of detector 300 is therefore proportional to the dotproduct of the incident light and the function effected by opticalfilter 294.

Although modulation density is used as the variable in the embodimentillustrated in FIG. 24D, it should be understood that pitch may also beused.

IR Up-Conversion Materials

Direct detection of infrared light is sometimes difficult when usingvisible light detectors. There are certain well-known materials that,however, when energized by a laser, receive infrared photons and releasephotons of visible light. These materials are used in infrared lightdetectors to receive infrared light. Their visible light output isproportional to the amount of received infrared light and is directed toa visible light detector. Thus, the output of the visible light detectoris proportional to the input infrared light.

This combination of devices may be used as a detector in the opticalfilter systems discussed herein to facilitate the use of these systemsfor IR chemical imaging. Referring to FIG. 25, an optical filter 306filters light 308 from a sample (not shown) and outputs the filteredlight to an IR conversion unit 308 energized by a laser 310. Theintensity of the visible light 312 output by unit 308 is proportional tothe infrared light of incident light 314. Accordingly, the output signal316 of a visible light detector 318 is a measure of the infrared lightcontent of incident light 314.

While preferred embodiments of the invention have been described above,it should be understood that any and all equivalent realizations of thepresent invention are included within the scope and spirit thereof.Thus, the embodiments depicted are presented by way of example only andare not intended as limitations upon the present invention. Therefore,it is contemplated that any and all such embodiments are included in thepresent invention as may fall within the scope of the appended claims.

1. A method of adjusting the intensity of light in an opticalspectroscopy system from a light source that emits light having anexpected average wavelength spectrum to maintain the reliability of alight signal from said light source, said method comprising the stepsof: modulating a first said light signal by an optical filter configuredto weight the intensity of said first light signal by wavelengthaccording to a regression vector that identifies a difference betweensaid average spectrum and a wavelength spectrum of said first lightsignal from said light source; comparing the intensity of said modulatedfirst light signal to an intensity expected if said wavelength spectrumof said first light signal equaled said average spectrum; and adjustinga power input to said light source responsively to said comparing stepto a degree so that a subsequent said light signal defines a wavelengthspectrum that is closer to said average spectrum, as measured by saidmodulating and comparing steps, than said wavelength spectrum of saidfirst light signal.
 2. The method as in claim 1, including collimatingsaid first light signal prior to said modulating step.
 3. The method asin claim 2, including bandpass filtering said first light signal priorto said modulating step to a wavelength range that at least includes anoperative wavelength range of said optical filter.
 4. For a light sourcein an optical spectroscopy system, a method of compensating for a changein a light signal, said method comprising the steps of: providing alight source that outputs a light signal having a wavelength spectrum;identifying a relationship between intensity of said light signal and adifference between said wavelength spectrum and an expected wavelengthspectrum of said light source; detecting intensity of said light signaloutput by said light source; and based on said relationship and saidintensity from said detecting step, modifying said wavelength spectrumin compensation for a change in said wavelength spectrum of said lightsignal.
 5. The method as in claim 4, wherein said identifying stepincludes defining a regression vector that identifies a differencebetween an expected average wavelength spectrum of said light source andsaid wavelength spectrum of said light signal, and wherein saiddetecting and modifying steps include prior to detecting intensity of afirst said light signal, modulating said first light signal by anoptical filter configured to weight the intensity of said first lightsignal by wavelength according to said regression vector; comparing theintensity of said modulated first light signal to an intensity expectedif said wavelength spectrum of first light signal equaled said averagespectrum; and adjusting a power input to said light source responsivelyto said comparing step to a degree so that a subsequent said lightsignal defines a wavelength spectrum that is closer to said averagespectrum, as measured by said modulating and comparing steps, than saidwavelength spectrum of said first light signal.
 6. The method as inclaim 4, wherein said light signal is non-monochromatic.
 7. For a lightsource in an optical spectroscopy system, a method of compensating forchange in a light signal, said method comprising the steps of: applyinga light signal from a light source to a measurement sample, wherein theentire wavelength range of said light signal applied to said measurementsample is simultaneously applied to said measurement sample; modulatingsaid light signal by an optical filter configured to weight theintensity of said light signal by wavelength according to apredetermined function that identifies a difference between an expectedspectrum of said light signal and an actual spectrum of said lightsignal; defining a relationship between change in spectral shape in saidwavelength range determined at said modulating step and change in inputpower to said light source; and based on said relationship, relating achange in said spectral shape to a modification in said input power andso modifying said input power in compensation for said change in saidspectral shape.